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-----------------------------------------------------------------------------This file contains a concatenation of the PCRE2 man pages, converted to plaintext format for ease of searching with a text editor, or for use on systemsthat do not have a man page processor. The small individual files that givesynopses of each function in the library have not been included. Neither hasthe pcre2demo program. There are separate text files for the pcre2grep andpcre2test commands.-----------------------------------------------------------------------------
PCRE2(3) Library Functions Manual PCRE2(3)
NAME PCRE2 - Perl-compatible regular expressions (revised API)
INTRODUCTION
PCRE2 is the name used for a revised API for the PCRE library, which is a set of functions, written in C, that implement regular expression pattern matching using the same syntax and semantics as Perl, with just a few differences. After nearly two decades, the limitations of the original API were making development increasingly difficult. The new API is more extensible, and it was simplified by abolishing the sepa- rate "study" optimizing function; in PCRE2, patterns are automatically optimized where possible. Since forking from PCRE1, the code has been extensively refactored and new features introduced. The old library is now obsolete and is no longer maintained.
As well as Perl-style regular expression patterns, some features that appeared in Python and the original PCRE before they appeared in Perl are available using the Python syntax. There is also some support for one or two .NET and Oniguruma syntax items, and there are options for requesting some minor changes that give better ECMAScript (aka JavaScript) compatibility.
The source code for PCRE2 can be compiled to support strings of 8-bit, 16-bit, or 32-bit code units, which means that up to three separate li- braries may be installed, one for each code unit size. The size of code unit is not related to the bit size of the underlying hardware. In a 64-bit environment that also supports 32-bit applications, versions of PCRE2 that are compiled in both 64-bit and 32-bit modes may be needed.
The original work to extend PCRE to 16-bit and 32-bit code units was done by Zoltan Herczeg and Christian Persch, respectively. In all three cases, strings can be interpreted either as one character per code unit, or as UTF-encoded Unicode, with support for Unicode general cate- gory properties. Unicode support is optional at build time (but is the default). However, processing strings as UTF code units must be enabled explicitly at run time. The version of Unicode in use can be discovered by running
pcre2test -C
The three libraries contain identical sets of functions, with names ending in _8, _16, or _32, respectively (for example, pcre2_com- pile_8()). However, by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or 32, a program that uses just one code unit width can be written using generic names such as pcre2_compile(), and the documentation is written assuming that this is the case.
In addition to the Perl-compatible matching function, PCRE2 contains an alternative function that matches the same compiled patterns in a dif- ferent way. In certain circumstances, the alternative function has some advantages. For a discussion of the two matching algorithms, see the pcre2matching page.
Details of exactly which Perl regular expression features are and are not supported by PCRE2 are given in separate documents. See the pcre2pattern and pcre2compat pages. There is a syntax summary in the pcre2syntax page.
Some features of PCRE2 can be included, excluded, or changed when the library is built. The pcre2_config() function makes it possible for a client to discover which features are available. The features them- selves are described in the pcre2build page. Documentation about build- ing PCRE2 for various operating systems can be found in the README and NON-AUTOTOOLS_BUILD files in the source distribution.
The libraries contains a number of undocumented internal functions and data tables that are used by more than one of the exported external functions, but which are not intended for use by external callers. Their names all begin with "_pcre2", which hopefully will not provoke any name clashes. In some environments, it is possible to control which external symbols are exported when a shared library is built, and in these cases the undocumented symbols are not exported.
SECURITY CONSIDERATIONS
If you are using PCRE2 in a non-UTF application that permits users to supply arbitrary patterns for compilation, you should be aware of a feature that allows users to turn on UTF support from within a pattern. For example, an 8-bit pattern that begins with "(*UTF)" turns on UTF-8 mode, which interprets patterns and subjects as strings of UTF-8 code units instead of individual 8-bit characters. This causes both the pat- tern and any data against which it is matched to be checked for UTF-8 validity. If the data string is very long, such a check might use suf- ficiently many resources as to cause your application to lose perfor- mance.
One way of guarding against this possibility is to use the pcre2_pat- tern_info() function to check the compiled pattern's options for PCRE2_UTF. Alternatively, you can set the PCRE2_NEVER_UTF option when calling pcre2_compile(). This causes a compile time error if the pat- tern contains a UTF-setting sequence.
The use of Unicode properties for character types such as \d can also be enabled from within the pattern, by specifying "(*UCP)". This fea- ture can be disallowed by setting the PCRE2_NEVER_UCP option.
If your application is one that supports UTF, be aware that validity checking can take time. If the same data string is to be matched many times, you can use the PCRE2_NO_UTF_CHECK option for the second and subsequent matches to avoid running redundant checks.
The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead to problems, because it may leave the current matching point in the middle of a multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C op- tion can be used by an application to lock out the use of \C, causing a compile-time error if it is encountered. It is also possible to build PCRE2 with the use of \C permanently disabled.
Another way that performance can be hit is by running a pattern that has a very large search tree against a string that will never match. Nested unlimited repeats in a pattern are a common example. PCRE2 pro- vides some protection against this: see the pcre2_set_match_limit() function in the pcre2api page. There is a similar function called pcre2_set_depth_limit() that can be used to restrict the amount of mem- ory that is used.
USER DOCUMENTATION
The user documentation for PCRE2 comprises a number of different sec- tions. In the "man" format, each of these is a separate "man page". In the HTML format, each is a separate page, linked from the index page. In the plain text format, the descriptions of the pcre2grep and pcre2test programs are in files called pcre2grep.txt and pcre2test.txt, respectively. The remaining sections, except for the pcre2demo section (which is a program listing), and the short pages for individual func- tions, are concatenated in pcre2.txt, for ease of searching. The sec- tions are as follows:
pcre2 this document pcre2-config show PCRE2 installation configuration information pcre2api details of PCRE2's native C API pcre2build building PCRE2 pcre2callout details of the pattern callout feature pcre2compat discussion of Perl compatibility pcre2convert details of pattern conversion functions pcre2demo a demonstration C program that uses PCRE2 pcre2grep description of the pcre2grep command (8-bit only) pcre2jit discussion of just-in-time optimization support pcre2limits details of size and other limits pcre2matching discussion of the two matching algorithms pcre2partial details of the partial matching facility pcre2pattern syntax and semantics of supported regular expression patterns pcre2perform discussion of performance issues pcre2posix the POSIX-compatible C API for the 8-bit library pcre2sample discussion of the pcre2demo program pcre2serialize details of pattern serialization pcre2syntax quick syntax reference pcre2test description of the pcre2test command pcre2unicode discussion of Unicode and UTF support
In the "man" and HTML formats, there is also a short page for each C library function, listing its arguments and results.
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
Putting an actual email address here is a spam magnet. If you want to email me, use my two names separated by a dot at gmail.com.
REVISION
Last updated: 27 August 2021 Copyright (c) 1997-2021 University of Cambridge.
PCRE2 10.38 27 August 2021 PCRE2(3)------------------------------------------------------------------------------
PCRE2API(3) Library Functions Manual PCRE2API(3)
NAME PCRE2 - Perl-compatible regular expressions (revised API)
#include <pcre2.h>
PCRE2 is a new API for PCRE, starting at release 10.0. This document contains a description of all its native functions. See the pcre2 docu- ment for an overview of all the PCRE2 documentation.
PCRE2 NATIVE API BASIC FUNCTIONS
pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length, uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset, pcre2_compile_context *ccontext);
void pcre2_code_free(pcre2_code *code);
pcre2_match_data *pcre2_match_data_create(uint32_t ovecsize, pcre2_general_context *gcontext);
pcre2_match_data *pcre2_match_data_create_from_pattern( const pcre2_code *code, pcre2_general_context *gcontext);
int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext);
int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext, int *workspace, PCRE2_SIZE wscount);
void pcre2_match_data_free(pcre2_match_data *match_data);
PCRE2 NATIVE API AUXILIARY MATCH FUNCTIONS
PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data);
PCRE2_SIZE pcre2_get_match_data_size(pcre2_match_data *match_data);
PCRE2_SIZE pcre2_get_match_data_heapframes_size( pcre2_match_data *match_data);
uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data);
PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data);
PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data);
PCRE2 NATIVE API GENERAL CONTEXT FUNCTIONS
pcre2_general_context *pcre2_general_context_create( void *(*private_malloc)(PCRE2_SIZE, void *), void (*private_free)(void *, void *), void *memory_data);
pcre2_general_context *pcre2_general_context_copy( pcre2_general_context *gcontext);
void pcre2_general_context_free(pcre2_general_context *gcontext);
PCRE2 NATIVE API COMPILE CONTEXT FUNCTIONS
pcre2_compile_context *pcre2_compile_context_create( pcre2_general_context *gcontext);
pcre2_compile_context *pcre2_compile_context_copy( pcre2_compile_context *ccontext);
void pcre2_compile_context_free(pcre2_compile_context *ccontext);
int pcre2_set_bsr(pcre2_compile_context *ccontext, uint32_t value);
int pcre2_set_character_tables(pcre2_compile_context *ccontext, const uint8_t *tables);
int pcre2_set_compile_extra_options(pcre2_compile_context *ccontext, uint32_t extra_options);
int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext, PCRE2_SIZE value);
int pcre2_set_max_pattern_compiled_length( pcre2_compile_context *ccontext, PCRE2_SIZE value);
int pcre2_set_max_varlookbehind(pcre2_compile_contest *ccontext, uint32_t value);
int pcre2_set_newline(pcre2_compile_context *ccontext, uint32_t value);
int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext, uint32_t value);
int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext, int (*guard_function)(uint32_t, void *), void *user_data);
PCRE2 NATIVE API MATCH CONTEXT FUNCTIONS
pcre2_match_context *pcre2_match_context_create( pcre2_general_context *gcontext);
pcre2_match_context *pcre2_match_context_copy( pcre2_match_context *mcontext);
void pcre2_match_context_free(pcre2_match_context *mcontext);
int pcre2_set_callout(pcre2_match_context *mcontext, int (*callout_function)(pcre2_callout_block *, void *), void *callout_data);
int pcre2_set_substitute_callout(pcre2_match_context *mcontext, int (*callout_function)(pcre2_substitute_callout_block *, void *), void *callout_data);
int pcre2_set_offset_limit(pcre2_match_context *mcontext, PCRE2_SIZE value);
int pcre2_set_heap_limit(pcre2_match_context *mcontext, uint32_t value);
int pcre2_set_match_limit(pcre2_match_context *mcontext, uint32_t value);
int pcre2_set_depth_limit(pcre2_match_context *mcontext, uint32_t value);
PCRE2 NATIVE API STRING EXTRACTION FUNCTIONS
int pcre2_substring_copy_byname(pcre2_match_data *match_data, PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen);
int pcre2_substring_copy_bynumber(pcre2_match_data *match_data, uint32_t number, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen);
void pcre2_substring_free(PCRE2_UCHAR *buffer);
int pcre2_substring_get_byname(pcre2_match_data *match_data, PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen);
int pcre2_substring_get_bynumber(pcre2_match_data *match_data, uint32_t number, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen);
int pcre2_substring_length_byname(pcre2_match_data *match_data, PCRE2_SPTR name, PCRE2_SIZE *length);
int pcre2_substring_length_bynumber(pcre2_match_data *match_data, uint32_t number, PCRE2_SIZE *length);
int pcre2_substring_nametable_scan(const pcre2_code *code, PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last);
int pcre2_substring_number_from_name(const pcre2_code *code, PCRE2_SPTR name);
void pcre2_substring_list_free(PCRE2_UCHAR **list);
int pcre2_substring_list_get(pcre2_match_data *match_data, PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr);
PCRE2 NATIVE API STRING SUBSTITUTION FUNCTION
int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext, PCRE2_SPTR replacementz, PCRE2_SIZE rlength, PCRE2_UCHAR *outputbuffer, PCRE2_SIZE *outlengthptr);
PCRE2 NATIVE API JIT FUNCTIONS
int pcre2_jit_compile(pcre2_code *code, uint32_t options);
int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext);
void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);
pcre2_jit_stack *pcre2_jit_stack_create(size_t startsize, size_t maxsize, pcre2_general_context *gcontext);
void pcre2_jit_stack_assign(pcre2_match_context *mcontext, pcre2_jit_callback callback_function, void *callback_data);
void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack);
PCRE2 NATIVE API SERIALIZATION FUNCTIONS
int32_t pcre2_serialize_decode(pcre2_code **codes, int32_t number_of_codes, const uint8_t *bytes, pcre2_general_context *gcontext);
int32_t pcre2_serialize_encode(const pcre2_code **codes, int32_t number_of_codes, uint8_t **serialized_bytes, PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext);
void pcre2_serialize_free(uint8_t *bytes);
int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes);
PCRE2 NATIVE API AUXILIARY FUNCTIONS
pcre2_code *pcre2_code_copy(const pcre2_code *code);
pcre2_code *pcre2_code_copy_with_tables(const pcre2_code *code);
int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer, PCRE2_SIZE bufflen);
const uint8_t *pcre2_maketables(pcre2_general_context *gcontext);
void pcre2_maketables_free(pcre2_general_context *gcontext, const uint8_t *tables);
int pcre2_pattern_info(const pcre2_code *code, uint32_t what, void *where);
int pcre2_callout_enumerate(const pcre2_code *code, int (*callback)(pcre2_callout_enumerate_block *, void *), void *user_data);
int pcre2_config(uint32_t what, void *where);
PCRE2 NATIVE API OBSOLETE FUNCTIONS
int pcre2_set_recursion_limit(pcre2_match_context *mcontext, uint32_t value);
int pcre2_set_recursion_memory_management( pcre2_match_context *mcontext, void *(*private_malloc)(size_t, void *), void (*private_free)(void *, void *), void *memory_data);
These functions became obsolete at release 10.30 and are retained only for backward compatibility. They should not be used in new code. The first is replaced by pcre2_set_depth_limit(); the second is no longer needed and has no effect (it always returns zero).
PCRE2 EXPERIMENTAL PATTERN CONVERSION FUNCTIONS
pcre2_convert_context *pcre2_convert_context_create( pcre2_general_context *gcontext);
pcre2_convert_context *pcre2_convert_context_copy( pcre2_convert_context *cvcontext);
void pcre2_convert_context_free(pcre2_convert_context *cvcontext);
int pcre2_set_glob_escape(pcre2_convert_context *cvcontext, uint32_t escape_char);
int pcre2_set_glob_separator(pcre2_convert_context *cvcontext, uint32_t separator_char);
int pcre2_pattern_convert(PCRE2_SPTR pattern, PCRE2_SIZE length, uint32_t options, PCRE2_UCHAR **buffer, PCRE2_SIZE *blength, pcre2_convert_context *cvcontext);
void pcre2_converted_pattern_free(PCRE2_UCHAR *converted_pattern);
These functions provide a way of converting non-PCRE2 patterns into patterns that can be processed by pcre2_compile(). This facility is ex- perimental and may be changed in future releases. At present, "globs" and POSIX basic and extended patterns can be converted. Details are given in the pcre2convert documentation.
PCRE2 8-BIT, 16-BIT, AND 32-BIT LIBRARIES
There are three PCRE2 libraries, supporting 8-bit, 16-bit, and 32-bit code units, respectively. However, there is just one header file, pcre2.h. This contains the function prototypes and other definitions for all three libraries. One, two, or all three can be installed simul- taneously. On Unix-like systems the libraries are called libpcre2-8, libpcre2-16, and libpcre2-32, and they can also co-exist with the orig- inal PCRE libraries. Every PCRE2 function comes in three different forms, one for each library, for example:
pcre2_compile_8() pcre2_compile_16() pcre2_compile_32()
There are also three different sets of data types:
PCRE2_UCHAR8, PCRE2_UCHAR16, PCRE2_UCHAR32 PCRE2_SPTR8, PCRE2_SPTR16, PCRE2_SPTR32
The UCHAR types define unsigned code units of the appropriate widths. For example, PCRE2_UCHAR16 is usually defined as `uint16_t'. The SPTR types are pointers to constants of the equivalent UCHAR types, that is, they are pointers to vectors of unsigned code units.
Character strings are passed to a PCRE2 library as sequences of un- signed integers in code units of the appropriate width. The length of a string may be given as a number of code units, or the string may be specified as zero-terminated.
Many applications use only one code unit width. For their convenience, macros are defined whose names are the generic forms such as pcre2_com- pile() and PCRE2_SPTR. These macros use the value of the macro PCRE2_CODE_UNIT_WIDTH to generate the appropriate width-specific func- tion and macro names. PCRE2_CODE_UNIT_WIDTH is not defined by default. An application must define it to be 8, 16, or 32 before including pcre2.h in order to make use of the generic names.
Applications that use more than one code unit width can be linked with more than one PCRE2 library, but must define PCRE2_CODE_UNIT_WIDTH to be 0 before including pcre2.h, and then use the real function names. Any code that is to be included in an environment where the value of PCRE2_CODE_UNIT_WIDTH is unknown should also use the real function names. (Unfortunately, it is not possible in C code to save and restore the value of a macro.)
If PCRE2_CODE_UNIT_WIDTH is not defined before including pcre2.h, a compiler error occurs.
When using multiple libraries in an application, you must take care when processing any particular pattern to use only functions from a single library. For example, if you want to run a match using a pat- tern that was compiled with pcre2_compile_16(), you must do so with pcre2_match_16(), not pcre2_match_8() or pcre2_match_32().
In the function summaries above, and in the rest of this document and other PCRE2 documents, functions and data types are described using their generic names, without the _8, _16, or _32 suffix.
PCRE2 API OVERVIEW
PCRE2 has its own native API, which is described in this document. There are also some wrapper functions for the 8-bit library that corre- spond to the POSIX regular expression API, but they do not give access to all the functionality of PCRE2 and they are not thread-safe. They are described in the pcre2posix documentation. Both these APIs define a set of C function calls.
The native API C data types, function prototypes, option values, and error codes are defined in the header file pcre2.h, which also contains definitions of PCRE2_MAJOR and PCRE2_MINOR, the major and minor release numbers for the library. Applications can use these to include support for different releases of PCRE2.
In a Windows environment, if you want to statically link an application program against a non-dll PCRE2 library, you must define PCRE2_STATIC before including pcre2.h.
The functions pcre2_compile() and pcre2_match() are used for compiling and matching regular expressions in a Perl-compatible manner. A sample program that demonstrates the simplest way of using them is provided in the file called pcre2demo.c in the PCRE2 source distribution. A listing of this program is given in the pcre2demo documentation, and the pcre2sample documentation describes how to compile and run it.
The compiling and matching functions recognize various options that are passed as bits in an options argument. There are also some more compli- cated parameters such as custom memory management functions and re- source limits that are passed in "contexts" (which are just memory blocks, described below). Simple applications do not need to make use of contexts.
Just-in-time (JIT) compiler support is an optional feature of PCRE2 that can be built in appropriate hardware environments. It greatly speeds up the matching performance of many patterns. Programs can re- quest that it be used if available by calling pcre2_jit_compile() after a pattern has been successfully compiled by pcre2_compile(). This does nothing if JIT support is not available.
More complicated programs might need to make use of the specialist functions pcre2_jit_stack_create(), pcre2_jit_stack_free(), and pcre2_jit_stack_assign() in order to control the JIT code's memory us- age.
JIT matching is automatically used by pcre2_match() if it is available, unless the PCRE2_NO_JIT option is set. There is also a direct interface for JIT matching, which gives improved performance at the expense of less sanity checking. The JIT-specific functions are discussed in the pcre2jit documentation.
A second matching function, pcre2_dfa_match(), which is not Perl-com- patible, is also provided. This uses a different algorithm for the matching. The alternative algorithm finds all possible matches (at a given point in the subject), and scans the subject just once (unless there are lookaround assertions). However, this algorithm does not re- turn captured substrings. A description of the two matching algorithms and their advantages and disadvantages is given in the pcre2matching documentation. There is no JIT support for pcre2_dfa_match().
In addition to the main compiling and matching functions, there are convenience functions for extracting captured substrings from a subject string that has been matched by pcre2_match(). They are:
pcre2_substring_copy_byname() pcre2_substring_copy_bynumber() pcre2_substring_get_byname() pcre2_substring_get_bynumber() pcre2_substring_list_get() pcre2_substring_length_byname() pcre2_substring_length_bynumber() pcre2_substring_nametable_scan() pcre2_substring_number_from_name()
pcre2_substring_free() and pcre2_substring_list_free() are also pro- vided, to free memory used for extracted strings. If either of these functions is called with a NULL argument, the function returns immedi- ately without doing anything.
The function pcre2_substitute() can be called to match a pattern and return a copy of the subject string with substitutions for parts that were matched.
Functions whose names begin with pcre2_serialize_ are used for saving compiled patterns on disc or elsewhere, and reloading them later.
Finally, there are functions for finding out information about a com- piled pattern (pcre2_pattern_info()) and about the configuration with which PCRE2 was built (pcre2_config()).
Functions with names ending with _free() are used for freeing memory blocks of various sorts. In all cases, if one of these functions is called with a NULL argument, it does nothing.
STRING LENGTHS AND OFFSETS
The PCRE2 API uses string lengths and offsets into strings of code units in several places. These values are always of type PCRE2_SIZE, which is an unsigned integer type, currently always defined as size_t. The largest value that can be stored in such a type (that is ~(PCRE2_SIZE)0) is reserved as a special indicator for zero-terminated strings and unset offsets. Therefore, the longest string that can be handled is one less than this maximum. Note that string lengths are al- ways given in code units. Only in the 8-bit library is such a length the same as the number of bytes in the string.
NEWLINES
PCRE2 supports five different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (line- feed) character, the two-character sequence CRLF, any of the three pre- ceding, or any Unicode newline sequence. The Unicode newline sequences are the three just mentioned, plus the single characters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS (paragraph separator, U+2029).
Each of the first three conventions is used by at least one operating system as its standard newline sequence. When PCRE2 is built, a default can be specified. If it is not, the default is set to LF, which is the Unix standard. However, the newline convention can be changed by an ap- plication when calling pcre2_compile(), or it can be specified by spe- cial text at the start of the pattern itself; this overrides any other settings. See the pcre2pattern page for details of the special charac- ter sequences.
In the PCRE2 documentation the word "newline" is used to mean "the character or pair of characters that indicate a line break". The choice of newline convention affects the handling of the dot, circumflex, and dollar metacharacters, the handling of #-comments in /x mode, and, when CRLF is a recognized line ending sequence, the match position advance- ment for a non-anchored pattern. There is more detail about this in the section on pcre2_match() options below.
The choice of newline convention does not affect the interpretation of the \n or \r escape sequences, nor does it affect what \R matches; this has its own separate convention.
MULTITHREADING
In a multithreaded application it is important to keep thread-specific data separate from data that can be shared between threads. The PCRE2 library code itself is thread-safe: it contains no static or global variables. The API is designed to be fairly simple for non-threaded ap- plications while at the same time ensuring that multithreaded applica- tions can use it.
There are several different blocks of data that are used to pass infor- mation between the application and the PCRE2 libraries.
The compiled pattern
A pointer to the compiled form of a pattern is returned to the user when pcre2_compile() is successful. The data in the compiled pattern is fixed, and does not change when the pattern is matched. Therefore, it is thread-safe, that is, the same compiled pattern can be used by more than one thread simultaneously. For example, an application can compile all its patterns at the start, before forking off multiple threads that use them. However, if the just-in-time (JIT) optimization feature is being used, it needs separate memory stack areas for each thread. See the pcre2jit documentation for more details.
In a more complicated situation, where patterns are compiled only when they are first needed, but are still shared between threads, pointers to compiled patterns must be protected from simultaneous writing by multiple threads. This is somewhat tricky to do correctly. If you know that writing to a pointer is atomic in your environment, you can use logic like this:
Get a read-only (shared) lock (mutex) for pointer if (pointer == NULL) { Get a write (unique) lock for pointer if (pointer == NULL) pointer = pcre2_compile(... } Release the lock Use pointer in pcre2_match()
Of course, testing for compilation errors should also be included in the code.
The reason for checking the pointer a second time is as follows: Sev- eral threads may have acquired the shared lock and tested the pointer for being NULL, but only one of them will be given the write lock, with the rest kept waiting. The winning thread will compile the pattern and store the result. After this thread releases the write lock, another thread will get it, and if it does not retest pointer for being NULL, will recompile the pattern and overwrite the pointer, creating a memory leak and possibly causing other issues.
In an environment where writing to a pointer may not be atomic, the above logic is not sufficient. The thread that is doing the compiling may be descheduled after writing only part of the pointer, which could cause other threads to use an invalid value. Instead of checking the pointer itself, a separate "pointer is valid" flag (that can be updated atomically) must be used:
Get a read-only (shared) lock (mutex) for pointer if (!pointer_is_valid) { Get a write (unique) lock for pointer if (!pointer_is_valid) { pointer = pcre2_compile(... pointer_is_valid = TRUE } } Release the lock Use pointer in pcre2_match()
If JIT is being used, but the JIT compilation is not being done immedi- ately (perhaps waiting to see if the pattern is used often enough), similar logic is required. JIT compilation updates a value within the compiled code block, so a thread must gain unique write access to the pointer before calling pcre2_jit_compile(). Alternatively, pcre2_code_copy() or pcre2_code_copy_with_tables() can be used to ob- tain a private copy of the compiled code before calling the JIT com- piler.
Context blocks
The next main section below introduces the idea of "contexts" in which PCRE2 functions are called. A context is nothing more than a collection of parameters that control the way PCRE2 operates. Grouping a number of parameters together in a context is a convenient way of passing them to a PCRE2 function without using lots of arguments. The parameters that are stored in contexts are in some sense "advanced features" of the API. Many straightforward applications will not need to use contexts.
In a multithreaded application, if the parameters in a context are val- ues that are never changed, the same context can be used by all the threads. However, if any thread needs to change any value in a context, it must make its own thread-specific copy.
Match blocks
The matching functions need a block of memory for storing the results of a match. This includes details of what was matched, as well as addi- tional information such as the name of a (*MARK) setting. Each thread must provide its own copy of this memory.
PCRE2 CONTEXTS
Some PCRE2 functions have a lot of parameters, many of which are used only by specialist applications, for example, those that use custom memory management or non-standard character tables. To keep function argument lists at a reasonable size, and at the same time to keep the API extensible, "uncommon" parameters are passed to certain functions in a context instead of directly. A context is just a block of memory that holds the parameter values. Applications that do not need to ad- just any of the context parameters can pass NULL when a context pointer is required.
There are three different types of context: a general context that is relevant for several PCRE2 operations, a compile-time context, and a match-time context.
The general context
At present, this context just contains pointers to (and data for) ex- ternal memory management functions that are called from several places in the PCRE2 library. The context is named `general' rather than specifically `memory' because in future other fields may be added. If you do not want to supply your own custom memory management functions, you do not need to bother with a general context. A general context is created by:
pcre2_general_context *pcre2_general_context_create( void *(*private_malloc)(PCRE2_SIZE, void *), void (*private_free)(void *, void *), void *memory_data);
The two function pointers specify custom memory management functions, whose prototypes are:
void *private_malloc(PCRE2_SIZE, void *); void private_free(void *, void *);
Whenever code in PCRE2 calls these functions, the final argument is the value of memory_data. Either of the first two arguments of the creation function may be NULL, in which case the system memory management func- tions malloc() and free() are used. (This is not currently useful, as there are no other fields in a general context, but in future there might be.) The private_malloc() function is used (if supplied) to ob- tain memory for storing the context, and all three values are saved as part of the context.
Whenever PCRE2 creates a data block of any kind, the block contains a pointer to the free() function that matches the malloc() function that was used. When the time comes to free the block, this function is called.
A general context can be copied by calling:
pcre2_general_context *pcre2_general_context_copy( pcre2_general_context *gcontext);
The memory used for a general context should be freed by calling:
void pcre2_general_context_free(pcre2_general_context *gcontext);
If this function is passed a NULL argument, it returns immediately without doing anything.
The compile context
A compile context is required if you want to provide an external func- tion for stack checking during compilation or to change the default values of any of the following compile-time parameters:
What \R matches (Unicode newlines or CR, LF, CRLF only) PCRE2's character tables The newline character sequence The compile time nested parentheses limit The maximum length of the pattern string The extra options bits (none set by default)
A compile context is also required if you are using custom memory man- agement. If none of these apply, just pass NULL as the context argu- ment of pcre2_compile().
A compile context is created, copied, and freed by the following func- tions:
pcre2_compile_context *pcre2_compile_context_create( pcre2_general_context *gcontext);
pcre2_compile_context *pcre2_compile_context_copy( pcre2_compile_context *ccontext);
void pcre2_compile_context_free(pcre2_compile_context *ccontext);
A compile context is created with default values for its parameters. These can be changed by calling the following functions, which return 0 on success, or PCRE2_ERROR_BADDATA if invalid data is detected.
int pcre2_set_bsr(pcre2_compile_context *ccontext, uint32_t value);
The value must be PCRE2_BSR_ANYCRLF, to specify that \R matches only CR, LF, or CRLF, or PCRE2_BSR_UNICODE, to specify that \R matches any Unicode line ending sequence. The value is used by the JIT compiler and by the two interpreted matching functions, pcre2_match() and pcre2_dfa_match().
int pcre2_set_character_tables(pcre2_compile_context *ccontext, const uint8_t *tables);
The value must be the result of a call to pcre2_maketables(), whose only argument is a general context. This function builds a set of char- acter tables in the current locale.
int pcre2_set_compile_extra_options(pcre2_compile_context *ccontext, uint32_t extra_options);
As PCRE2 has developed, almost all the 32 option bits that are avail- able in the options argument of pcre2_compile() have been used up. To avoid running out, the compile context contains a set of extra option bits which are used for some newer, assumed rarer, options. This func- tion sets those bits. It always sets all the bits (either on or off). It does not modify any existing setting. The available options are de- fined in the section entitled "Extra compile options" below.
int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext, PCRE2_SIZE value);
This sets a maximum length, in code units, for any pattern string that is compiled with this context. If the pattern is longer, an error is generated. This facility is provided so that applications that accept patterns from external sources can limit their size. The default is the largest number that a PCRE2_SIZE variable can hold, which is effec- tively unlimited.
int pcre2_set_max_pattern_compiled_length( pcre2_compile_context *ccontext, PCRE2_SIZE value);
This sets a maximum size, in bytes, for the memory needed to hold the compiled version of a pattern that is compiled with this context. If the pattern needs more memory, an error is generated. This facility is provided so that applications that accept patterns from external sources can limit the amount of memory they use. The default is the largest number that a PCRE2_SIZE variable can hold, which is effec- tively unlimited.
int pcre2_set_max_varlookbehind(pcre2_compile_contest *ccontext, uint32_t value);
This sets a maximum length for the number of characters matched by a variable-length lookbehind assertion. The default is set when PCRE2 is built, with the ultimate default being 255, the same as Perl. Lookbe- hind assertions without a bounding length are not supported.
int pcre2_set_newline(pcre2_compile_context *ccontext, uint32_t value);
This specifies which characters or character sequences are to be recog- nized as newlines. The value must be one of PCRE2_NEWLINE_CR (carriage return only), PCRE2_NEWLINE_LF (linefeed only), PCRE2_NEWLINE_CRLF (the two-character sequence CR followed by LF), PCRE2_NEWLINE_ANYCRLF (any of the above), PCRE2_NEWLINE_ANY (any Unicode newline sequence), or PCRE2_NEWLINE_NUL (the NUL character, that is a binary zero).
A pattern can override the value set in the compile context by starting with a sequence such as (*CRLF). See the pcre2pattern page for details.
When a pattern is compiled with the PCRE2_EXTENDED or PCRE2_EX- TENDED_MORE option, the newline convention affects the recognition of the end of internal comments starting with #. The value is saved with the compiled pattern for subsequent use by the JIT compiler and by the two interpreted matching functions, pcre2_match() and pcre2_dfa_match().
int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext, uint32_t value);
This parameter adjusts the limit, set when PCRE2 is built (default 250), on the depth of parenthesis nesting in a pattern. This limit stops rogue patterns using up too much system stack when being com- piled. The limit applies to parentheses of all kinds, not just captur- ing parentheses.
int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext, int (*guard_function)(uint32_t, void *), void *user_data);
There is at least one application that runs PCRE2 in threads with very limited system stack, where running out of stack is to be avoided at all costs. The parenthesis limit above cannot take account of how much stack is actually available during compilation. For a finer control, you can supply a function that is called whenever pcre2_compile() starts to compile a parenthesized part of a pattern. This function can check the actual stack size (or anything else that it wants to, of course).
The first argument to the callout function gives the current depth of nesting, and the second is user data that is set up by the last argu- ment of pcre2_set_compile_recursion_guard(). The callout function should return zero if all is well, or non-zero to force an error.
The match context
A match context is required if you want to:
Set up a callout function Set an offset limit for matching an unanchored pattern Change the limit on the amount of heap used when matching Change the backtracking match limit Change the backtracking depth limit Set custom memory management specifically for the match
If none of these apply, just pass NULL as the context argument of pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match().
A match context is created, copied, and freed by the following func- tions:
pcre2_match_context *pcre2_match_context_create( pcre2_general_context *gcontext);
pcre2_match_context *pcre2_match_context_copy( pcre2_match_context *mcontext);
void pcre2_match_context_free(pcre2_match_context *mcontext);
A match context is created with default values for its parameters. These can be changed by calling the following functions, which return 0 on success, or PCRE2_ERROR_BADDATA if invalid data is detected.
int pcre2_set_callout(pcre2_match_context *mcontext, int (*callout_function)(pcre2_callout_block *, void *), void *callout_data);
This sets up a callout function for PCRE2 to call at specified points during a matching operation. Details are given in the pcre2callout doc- umentation.
int pcre2_set_substitute_callout(pcre2_match_context *mcontext, int (*callout_function)(pcre2_substitute_callout_block *, void *), void *callout_data);
This sets up a callout function for PCRE2 to call after each substitu- tion made by pcre2_substitute(). Details are given in the section enti- tled "Creating a new string with substitutions" below.
int pcre2_set_offset_limit(pcre2_match_context *mcontext, PCRE2_SIZE value);
The offset_limit parameter limits how far an unanchored search can ad- vance in the subject string. The default value is PCRE2_UNSET. The pcre2_match() and pcre2_dfa_match() functions return PCRE2_ERROR_NO- MATCH if a match with a starting point before or at the given offset is not found. The pcre2_substitute() function makes no more substitutions.
For example, if the pattern /abc/ is matched against "123abc" with an offset limit less than 3, the result is PCRE2_ERROR_NOMATCH. A match can never be found if the startoffset argument of pcre2_match(), pcre2_dfa_match(), or pcre2_substitute() is greater than the offset limit set in the match context.
When using this facility, you must set the PCRE2_USE_OFFSET_LIMIT op- tion when calling pcre2_compile() so that when JIT is in use, different code can be compiled. If a match is started with a non-default match limit when PCRE2_USE_OFFSET_LIMIT is not set, an error is generated.
The offset limit facility can be used to track progress when searching large subject strings or to limit the extent of global substitutions. See also the PCRE2_FIRSTLINE option, which requires a match to start before or at the first newline that follows the start of matching in the subject. If this is set with an offset limit, a match must occur in the first line and also within the offset limit. In other words, whichever limit comes first is used.
int pcre2_set_heap_limit(pcre2_match_context *mcontext, uint32_t value);
The heap_limit parameter specifies, in units of kibibytes (1024 bytes), the maximum amount of heap memory that pcre2_match() may use to hold backtracking information when running an interpretive match. This limit also applies to pcre2_dfa_match(), which may use the heap when process- ing patterns with a lot of nested pattern recursion or lookarounds or atomic groups. This limit does not apply to matching with the JIT opti- mization, which has its own memory control arrangements (see the pcre2jit documentation for more details). If the limit is reached, the negative error code PCRE2_ERROR_HEAPLIMIT is returned. The default limit can be set when PCRE2 is built; if it is not, the default is set very large and is essentially unlimited.
A value for the heap limit may also be supplied by an item at the start of a pattern of the form
(*LIMIT_HEAP=ddd)
where ddd is a decimal number. However, such a setting is ignored un- less ddd is less than the limit set by the caller of pcre2_match() or, if no such limit is set, less than the default.
The pcre2_match() function always needs some heap memory, so setting a value of zero guarantees a "heap limit exceeded" error. Details of how pcre2_match() uses the heap are given in the pcre2perform documenta- tion.
For pcre2_dfa_match(), a vector on the system stack is used when pro- cessing pattern recursions, lookarounds, or atomic groups, and only if this is not big enough is heap memory used. In this case, setting a value of zero disables the use of the heap.
int pcre2_set_match_limit(pcre2_match_context *mcontext, uint32_t value);
The match_limit parameter provides a means of preventing PCRE2 from us- ing up too many computing resources when processing patterns that are not going to match, but which have a very large number of possibilities in their search trees. The classic example is a pattern that uses nested unlimited repeats.
There is an internal counter in pcre2_match() that is incremented each time round its main matching loop. If this value reaches the match limit, pcre2_match() returns the negative value PCRE2_ERROR_MATCHLIMIT. This has the effect of limiting the amount of backtracking that can take place. For patterns that are not anchored, the count restarts from zero for each position in the subject string. This limit also applies to pcre2_dfa_match(), though the counting is done in a different way.
When pcre2_match() is called with a pattern that was successfully processed by pcre2_jit_compile(), the way in which matching is executed is entirely different. However, there is still the possibility of run- away matching that goes on for a very long time, and so the match_limit value is also used in this case (but in a different way) to limit how long the matching can continue.
The default value for the limit can be set when PCRE2 is built; the de- fault is 10 million, which handles all but the most extreme cases. A value for the match limit may also be supplied by an item at the start of a pattern of the form
(*LIMIT_MATCH=ddd)
where ddd is a decimal number. However, such a setting is ignored un- less ddd is less than the limit set by the caller of pcre2_match() or pcre2_dfa_match() or, if no such limit is set, less than the default.
int pcre2_set_depth_limit(pcre2_match_context *mcontext, uint32_t value);
This parameter limits the depth of nested backtracking in pcre2_match(). Each time a nested backtracking point is passed, a new memory frame is used to remember the state of matching at that point. Thus, this parameter indirectly limits the amount of memory that is used in a match. However, because the size of each memory frame depends on the number of capturing parentheses, the actual memory limit varies from pattern to pattern. This limit was more useful in versions before 10.30, where function recursion was used for backtracking.
The depth limit is not relevant, and is ignored, when matching is done using JIT compiled code. However, it is supported by pcre2_dfa_match(), which uses it to limit the depth of nested internal recursive function calls that implement atomic groups, lookaround assertions, and pattern recursions. This limits, indirectly, the amount of system stack that is used. It was more useful in versions before 10.32, when stack memory was used for local workspace vectors for recursive function calls. From version 10.32, only local variables are allocated on the stack and as each call uses only a few hundred bytes, even a small stack can support quite a lot of recursion.
If the depth of internal recursive function calls is great enough, lo- cal workspace vectors are allocated on the heap from version 10.32 on- wards, so the depth limit also indirectly limits the amount of heap memory that is used. A recursive pattern such as /(.(?2))((?1)|)/, when matched to a very long string using pcre2_dfa_match(), can use a great deal of memory. However, it is probably better to limit heap usage di- rectly by calling pcre2_set_heap_limit().
The default value for the depth limit can be set when PCRE2 is built; if it is not, the default is set to the same value as the default for the match limit. If the limit is exceeded, pcre2_match() or pcre2_dfa_match() returns PCRE2_ERROR_DEPTHLIMIT. A value for the depth limit may also be supplied by an item at the start of a pattern of the form
(*LIMIT_DEPTH=ddd)
where ddd is a decimal number. However, such a setting is ignored un- less ddd is less than the limit set by the caller of pcre2_match() or pcre2_dfa_match() or, if no such limit is set, less than the default.
CHECKING BUILD-TIME OPTIONS
int pcre2_config(uint32_t what, void *where);
The function pcre2_config() makes it possible for a PCRE2 client to find the value of certain configuration parameters and to discover which optional features have been compiled into the PCRE2 library. The pcre2build documentation has more details about these features.
The first argument for pcre2_config() specifies which information is required. The second argument is a pointer to memory into which the in- formation is placed. If NULL is passed, the function returns the amount of memory that is needed for the requested information. For calls that return numerical values, the value is in bytes; when requesting these values, where should point to appropriately aligned memory. For calls that return strings, the required length is given in code units, not counting the terminating zero.
When requesting information, the returned value from pcre2_config() is non-negative on success, or the negative error code PCRE2_ERROR_BADOP- TION if the value in the first argument is not recognized. The follow- ing information is available:
PCRE2_CONFIG_BSR
The output is a uint32_t integer whose value indicates what character sequences the \R escape sequence matches by default. A value of PCRE2_BSR_UNICODE means that \R matches any Unicode line ending se- quence; a value of PCRE2_BSR_ANYCRLF means that \R matches only CR, LF, or CRLF. The default can be overridden when a pattern is compiled.
PCRE2_CONFIG_COMPILED_WIDTHS
The output is a uint32_t integer whose lower bits indicate which code unit widths were selected when PCRE2 was built. The 1-bit indicates 8-bit support, and the 2-bit and 4-bit indicate 16-bit and 32-bit sup- port, respectively.
PCRE2_CONFIG_DEPTHLIMIT
The output is a uint32_t integer that gives the default limit for the depth of nested backtracking in pcre2_match() or the depth of nested recursions, lookarounds, and atomic groups in pcre2_dfa_match(). Fur- ther details are given with pcre2_set_depth_limit() above.
PCRE2_CONFIG_HEAPLIMIT
The output is a uint32_t integer that gives, in kibibytes, the default limit for the amount of heap memory used by pcre2_match() or pcre2_dfa_match(). Further details are given with pcre2_set_heap_limit() above.
PCRE2_CONFIG_JIT
The output is a uint32_t integer that is set to one if support for just-in-time compiling is included in the library; otherwise it is set to zero. Note that having the support in the library does not guarantee that JIT will be used for any given match. See the pcre2jit documenta- tion for more details.
PCRE2_CONFIG_JITTARGET
The where argument should point to a buffer that is at least 48 code units long. (The exact length required can be found by calling pcre2_config() with where set to NULL.) The buffer is filled with a string that contains the name of the architecture for which the JIT compiler is configured, for example "x86 32bit (little endian + un- aligned)". If JIT support is not available, PCRE2_ERROR_BADOPTION is returned, otherwise the number of code units used is returned. This is the length of the string, plus one unit for the terminating zero.
PCRE2_CONFIG_LINKSIZE
The output is a uint32_t integer that contains the number of bytes used for internal linkage in compiled regular expressions. When PCRE2 is configured, the value can be set to 2, 3, or 4, with the default being 2. This is the value that is returned by pcre2_config(). However, when the 16-bit library is compiled, a value of 3 is rounded up to 4, and when the 32-bit library is compiled, internal linkages always use 4 bytes, so the configured value is not relevant.
The default value of 2 for the 8-bit and 16-bit libraries is sufficient for all but the most massive patterns, since it allows the size of the compiled pattern to be up to 65535 code units. Larger values allow larger regular expressions to be compiled by those two libraries, but at the expense of slower matching.
PCRE2_CONFIG_MATCHLIMIT
The output is a uint32_t integer that gives the default match limit for pcre2_match(). Further details are given with pcre2_set_match_limit() above.
PCRE2_CONFIG_NEWLINE
The output is a uint32_t integer whose value specifies the default character sequence that is recognized as meaning "newline". The values are:
PCRE2_NEWLINE_CR Carriage return (CR) PCRE2_NEWLINE_LF Linefeed (LF) PCRE2_NEWLINE_CRLF Carriage return, linefeed (CRLF) PCRE2_NEWLINE_ANY Any Unicode line ending PCRE2_NEWLINE_ANYCRLF Any of CR, LF, or CRLF PCRE2_NEWLINE_NUL The NUL character (binary zero)
The default should normally correspond to the standard sequence for your operating system.
PCRE2_CONFIG_NEVER_BACKSLASH_C
The output is a uint32_t integer that is set to one if the use of \C was permanently disabled when PCRE2 was built; otherwise it is set to zero.
PCRE2_CONFIG_PARENSLIMIT
The output is a uint32_t integer that gives the maximum depth of nest- ing of parentheses (of any kind) in a pattern. This limit is imposed to cap the amount of system stack used when a pattern is compiled. It is specified when PCRE2 is built; the default is 250. This limit does not take into account the stack that may already be used by the calling ap- plication. For finer control over compilation stack usage, see pcre2_set_compile_recursion_guard().
PCRE2_CONFIG_STACKRECURSE
This parameter is obsolete and should not be used in new code. The out- put is a uint32_t integer that is always set to zero.
PCRE2_CONFIG_TABLES_LENGTH
The output is a uint32_t integer that gives the length of PCRE2's char- acter processing tables in bytes. For details of these tables see the section on locale support below.
PCRE2_CONFIG_UNICODE_VERSION
The where argument should point to a buffer that is at least 24 code units long. (The exact length required can be found by calling pcre2_config() with where set to NULL.) If PCRE2 has been compiled without Unicode support, the buffer is filled with the text "Unicode not supported". Otherwise, the Unicode version string (for example, "8.0.0") is inserted. The number of code units used is returned. This is the length of the string plus one unit for the terminating zero.
PCRE2_CONFIG_UNICODE
The output is a uint32_t integer that is set to one if Unicode support is available; otherwise it is set to zero. Unicode support implies UTF support.
PCRE2_CONFIG_VERSION
The where argument should point to a buffer that is at least 24 code units long. (The exact length required can be found by calling pcre2_config() with where set to NULL.) The buffer is filled with the PCRE2 version string, zero-terminated. The number of code units used is returned. This is the length of the string plus one unit for the termi- nating zero.
COMPILING A PATTERN
pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length, uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset, pcre2_compile_context *ccontext);
void pcre2_code_free(pcre2_code *code);
pcre2_code *pcre2_code_copy(const pcre2_code *code);
pcre2_code *pcre2_code_copy_with_tables(const pcre2_code *code);
The pcre2_compile() function compiles a pattern into an internal form. The pattern is defined by a pointer to a string of code units and a length in code units. If the pattern is zero-terminated, the length can be specified as PCRE2_ZERO_TERMINATED. A NULL pattern pointer with a length of zero is treated as an empty string (NULL with a non-zero length causes an error return). The function returns a pointer to a block of memory that contains the compiled pattern and related data, or NULL if an error occurred.
If the compile context argument ccontext is NULL, memory for the com- piled pattern is obtained by calling malloc(). Otherwise, it is ob- tained from the same memory function that was used for the compile con- text. The caller must free the memory by calling pcre2_code_free() when it is no longer needed. If pcre2_code_free() is called with a NULL ar- gument, it returns immediately, without doing anything.
The function pcre2_code_copy() makes a copy of the compiled code in new memory, using the same memory allocator as was used for the original. However, if the code has been processed by the JIT compiler (see be- low), the JIT information cannot be copied (because it is position-de- pendent). The new copy can initially be used only for non-JIT match- ing, though it can be passed to pcre2_jit_compile() if required. If pcre2_code_copy() is called with a NULL argument, it returns NULL.
The pcre2_code_copy() function provides a way for individual threads in a multithreaded application to acquire a private copy of shared com- piled code. However, it does not make a copy of the character tables used by the compiled pattern; the new pattern code points to the same tables as the original code. (See "Locale Support" below for details of these character tables.) In many applications the same tables are used throughout, so this behaviour is appropriate. Nevertheless, there are occasions when a copy of a compiled pattern and the relevant tables are needed. The pcre2_code_copy_with_tables() provides this facility. Copies of both the code and the tables are made, with the new code pointing to the new tables. The memory for the new tables is automati- cally freed when pcre2_code_free() is called for the new copy of the compiled code. If pcre2_code_copy_with_tables() is called with a NULL argument, it returns NULL.
NOTE: When one of the matching functions is called, pointers to the compiled pattern and the subject string are set in the match data block so that they can be referenced by the substring extraction functions after a successful match. After running a match, you must not free a compiled pattern or a subject string until after all operations on the match data block have taken place, unless, in the case of the subject string, you have used the PCRE2_COPY_MATCHED_SUBJECT option, which is described in the section entitled "Option bits for pcre2_match()" be- low.
The options argument for pcre2_compile() contains various bit settings that affect the compilation. It should be zero if none of them are re- quired. The available options are described below. Some of them (in particular, those that are compatible with Perl, but some others as well) can also be set and unset from within the pattern (see the de- tailed description in the pcre2pattern documentation).
For those options that can be different in different parts of the pat- tern, the contents of the options argument specifies their settings at the start of compilation. The PCRE2_ANCHORED, PCRE2_ENDANCHORED, and PCRE2_NO_UTF_CHECK options can be set at the time of matching as well as at compile time.
Some additional options and less frequently required compile-time para- meters (for example, the newline setting) can be provided in a compile context (as described above).
If errorcode or erroroffset is NULL, pcre2_compile() returns NULL imme- diately. Otherwise, the variables to which these point are set to an error code and an offset (number of code units) within the pattern, re- spectively, when pcre2_compile() returns NULL because a compilation er- ror has occurred.
There are nearly 100 positive error codes that pcre2_compile() may re- turn if it finds an error in the pattern. There are also some negative error codes that are used for invalid UTF strings when validity check- ing is in force. These are the same as given by pcre2_match() and pcre2_dfa_match(), and are described in the pcre2unicode documentation. There is no separate documentation for the positive error codes, be- cause the textual error messages that are obtained by calling the pcre2_get_error_message() function (see "Obtaining a textual error mes- sage" below) should be self-explanatory. Macro names starting with PCRE2_ERROR_ are defined for both positive and negative error codes in pcre2.h. When compilation is successful errorcode is set to a value that returns the message "no error" if passed to pcre2_get_error_mes- sage().
The value returned in erroroffset is an indication of where in the pat- tern an error occurred. When there is no error, zero is returned. A non-zero value is not necessarily the furthest point in the pattern that was read. For example, after the error "lookbehind assertion is not fixed length", the error offset points to the start of the failing assertion. For an invalid UTF-8 or UTF-16 string, the offset is that of the first code unit of the failing character.
Some errors are not detected until the whole pattern has been scanned; in these cases, the offset passed back is the length of the pattern. Note that the offset is in code units, not characters, even in a UTF mode. It may sometimes point into the middle of a UTF-8 or UTF-16 char- acter.
This code fragment shows a typical straightforward call to pcre2_com- pile():
pcre2_code *re; PCRE2_SIZE erroffset; int errorcode; re = pcre2_compile( "^A.*Z", /* the pattern */ PCRE2_ZERO_TERMINATED, /* the pattern is zero-terminated */ 0, /* default options */ &errorcode, /* for error code */ &erroffset, /* for error offset */ NULL); /* no compile context */
Main compile options
The following names for option bits are defined in the pcre2.h header file:
PCRE2_ANCHORED
If this bit is set, the pattern is forced to be "anchored", that is, it is constrained to match only at the first matching point in the string that is being searched (the "subject string"). This effect can also be achieved by appropriate constructs in the pattern itself, which is the only way to do it in Perl.
PCRE2_ALLOW_EMPTY_CLASS
By default, for compatibility with Perl, a closing square bracket that immediately follows an opening one is treated as a data character for the class. When PCRE2_ALLOW_EMPTY_CLASS is set, it terminates the class, which therefore contains no characters and so can never match.
PCRE2_ALT_BSUX
This option request alternative handling of three escape sequences, which makes PCRE2's behaviour more like ECMAscript (aka JavaScript). When it is set:
(1) \U matches an upper case "U" character; by default \U causes a com- pile time error (Perl uses \U to upper case subsequent characters).
(2) \u matches a lower case "u" character unless it is followed by four hexadecimal digits, in which case the hexadecimal number defines the code point to match. By default, \u causes a compile time error (Perl uses it to upper case the following character).
(3) \x matches a lower case "x" character unless it is followed by two hexadecimal digits, in which case the hexadecimal number defines the code point to match. By default, as in Perl, a hexadecimal number is always expected after \x, but it may have zero, one, or two digits (so, for example, \xz matches a binary zero character followed by z).
ECMAscript 6 added additional functionality to \u. This can be accessed using the PCRE2_EXTRA_ALT_BSUX extra option (see "Extra compile op- tions" below). Note that this alternative escape handling applies only to patterns. Neither of these options affects the processing of re- placement strings passed to pcre2_substitute().
PCRE2_ALT_CIRCUMFLEX
In multiline mode (when PCRE2_MULTILINE is set), the circumflex metacharacter matches at the start of the subject (unless PCRE2_NOTBOL is set), and also after any internal newline. However, it does not match after a newline at the end of the subject, for compatibility with Perl. If you want a multiline circumflex also to match after a termi- nating newline, you must set PCRE2_ALT_CIRCUMFLEX.
PCRE2_ALT_VERBNAMES
By default, for compatibility with Perl, the name in any verb sequence such as (*MARK:NAME) is any sequence of characters that does not in- clude a closing parenthesis. The name is not processed in any way, and it is not possible to include a closing parenthesis in the name. How- ever, if the PCRE2_ALT_VERBNAMES option is set, normal backslash pro- cessing is applied to verb names and only an unescaped closing paren- thesis terminates the name. A closing parenthesis can be included in a name either as \) or between \Q and \E. If the PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set with PCRE2_ALT_VERBNAMES, unescaped whitespace in verb names is skipped and #-comments are recognized, ex- actly as in the rest of the pattern.
PCRE2_AUTO_CALLOUT
If this bit is set, pcre2_compile() automatically inserts callout items, all with number 255, before each pattern item, except immedi- ately before or after an explicit callout in the pattern. For discus- sion of the callout facility, see the pcre2callout documentation.
PCRE2_CASELESS
If this bit is set, letters in the pattern match both upper and lower case letters in the subject. It is equivalent to Perl's /i option, and it can be changed within a pattern by a (?i) option setting. If either PCRE2_UTF or PCRE2_UCP is set, Unicode properties are used for all characters with more than one other case, and for all characters whose code points are greater than U+007F. Note that there are two ASCII characters, K and S, that, in addition to their lower case ASCII equiv- alents, are case-equivalent with U+212A (Kelvin sign) and U+017F (long S) respectively. If you do not want this case equivalence, you can sup- press it by setting PCRE2_EXTRA_CASELESS_RESTRICT.
For lower valued characters with only one other case, a lookup table is used for speed. When neither PCRE2_UTF nor PCRE2_UCP is set, a lookup table is used for all code points less than 256, and higher code points (available only in 16-bit or 32-bit mode) are treated as not having an- other case.
PCRE2_DOLLAR_ENDONLY
If this bit is set, a dollar metacharacter in the pattern matches only at the end of the subject string. Without this option, a dollar also matches immediately before a newline at the end of the string (but not before any other newlines). The PCRE2_DOLLAR_ENDONLY option is ignored if PCRE2_MULTILINE is set. There is no equivalent to this option in Perl, and no way to set it within a pattern.
PCRE2_DOTALL
If this bit is set, a dot metacharacter in the pattern matches any character, including one that indicates a newline. However, it only ever matches one character, even if newlines are coded as CRLF. Without this option, a dot does not match when the current position in the sub- ject is at a newline. This option is equivalent to Perl's /s option, and it can be changed within a pattern by a (?s) option setting. A neg- ative class such as [^a] always matches newline characters, and the \N escape sequence always matches a non-newline character, independent of the setting of PCRE2_DOTALL.
PCRE2_DUPNAMES
If this bit is set, names used to identify capture groups need not be unique. This can be helpful for certain types of pattern when it is known that only one instance of the named group can ever be matched. There are more details of named capture groups below; see also the pcre2pattern documentation.
PCRE2_ENDANCHORED
If this bit is set, the end of any pattern match must be right at the end of the string being searched (the "subject string"). If the pattern match succeeds by reaching (*ACCEPT), but does not reach the end of the subject, the match fails at the current starting point. For unanchored patterns, a new match is then tried at the next starting point. How- ever, if the match succeeds by reaching the end of the pattern, but not the end of the subject, backtracking occurs and an alternative match may be found. Consider these two patterns:
.(*ACCEPT)|.. .|..
If matched against "abc" with PCRE2_ENDANCHORED set, the first matches "c" whereas the second matches "bc". The effect of PCRE2_ENDANCHORED can also be achieved by appropriate constructs in the pattern itself, which is the only way to do it in Perl.
For DFA matching with pcre2_dfa_match(), PCRE2_ENDANCHORED applies only to the first (that is, the longest) matched string. Other parallel matches, which are necessarily substrings of the first one, must obvi- ously end before the end of the subject.
PCRE2_EXTENDED
If this bit is set, most white space characters in the pattern are to- tally ignored except when escaped, inside a character class, or inside a \Q...\E sequence. However, white space is not allowed within se- quences such as (?> that introduce various parenthesized groups, nor within numerical quantifiers such as {1,3}. Ignorable white space is permitted between an item and a following quantifier and between a quantifier and a following + that indicates possessiveness. PCRE2_EX- TENDED is equivalent to Perl's /x option, and it can be changed within a pattern by a (?x) option setting.
When PCRE2 is compiled without Unicode support, PCRE2_EXTENDED recog- nizes as white space only those characters with code points less than 256 that are flagged as white space in its low-character table. The ta- ble is normally created by pcre2_maketables(), which uses the isspace() function to identify space characters. In most ASCII environments, the relevant characters are those with code points 0x0009 (tab), 0x000A (linefeed), 0x000B (vertical tab), 0x000C (formfeed), 0x000D (carriage return), and 0x0020 (space).
When PCRE2 is compiled with Unicode support, in addition to these char- acters, five more Unicode "Pattern White Space" characters are recog- nized by PCRE2_EXTENDED. These are U+0085 (next line), U+200E (left-to- right mark), U+200F (right-to-left mark), U+2028 (line separator), and U+2029 (paragraph separator). This set of characters is the same as recognized by Perl's /x option. Note that the horizontal and vertical space characters that are matched by the \h and \v escapes in patterns are a much bigger set.
As well as ignoring most white space, PCRE2_EXTENDED also causes char- acters between an unescaped # outside a character class and the next newline, inclusive, to be ignored, which makes it possible to include comments inside complicated patterns. Note that the end of this type of comment is a literal newline sequence in the pattern; escape sequences that happen to represent a newline do not count.
Which characters are interpreted as newlines can be specified by a set- ting in the compile context that is passed to pcre2_compile() or by a special sequence at the start of the pattern, as described in the sec- tion entitled "Newline conventions" in the pcre2pattern documentation. A default is defined when PCRE2 is built.
PCRE2_EXTENDED_MORE
This option has the effect of PCRE2_EXTENDED, but, in addition, un- escaped space and horizontal tab characters are ignored inside a char- acter class. Note: only these two characters are ignored, not the full set of pattern white space characters that are ignored outside a char- acter class. PCRE2_EXTENDED_MORE is equivalent to Perl's /xx option, and it can be changed within a pattern by a (?xx) option setting.
PCRE2_FIRSTLINE
If this option is set, the start of an unanchored pattern match must be before or at the first newline in the subject string following the start of matching, though the matched text may continue over the new- line. If startoffset is non-zero, the limiting newline is not necessar- ily the first newline in the subject. For example, if the subject string is "abc\nxyz" (where \n represents a single-character newline) a pattern match for "yz" succeeds with PCRE2_FIRSTLINE if startoffset is greater than 3. See also PCRE2_USE_OFFSET_LIMIT, which provides a more general limiting facility. If PCRE2_FIRSTLINE is set with an offset limit, a match must occur in the first line and also within the offset limit. In other words, whichever limit comes first is used. This option has no effect for anchored patterns.
PCRE2_LITERAL
If this option is set, all meta-characters in the pattern are disabled, and it is treated as a literal string. Matching literal strings with a regular expression engine is not the most efficient way of doing it. If you are doing a lot of literal matching and are worried about effi- ciency, you should consider using other approaches. The only other main options that are allowed with PCRE2_LITERAL are: PCRE2_ANCHORED, PCRE2_ENDANCHORED, PCRE2_AUTO_CALLOUT, PCRE2_CASELESS, PCRE2_FIRSTLINE, PCRE2_MATCH_INVALID_UTF, PCRE2_NO_START_OPTIMIZE, PCRE2_NO_UTF_CHECK, PCRE2_UTF, and PCRE2_USE_OFFSET_LIMIT. The extra options PCRE2_EX- TRA_MATCH_LINE and PCRE2_EXTRA_MATCH_WORD are also supported. Any other options cause an error.
PCRE2_MATCH_INVALID_UTF
This option forces PCRE2_UTF (see below) and also enables support for matching by pcre2_match() in subject strings that contain invalid UTF sequences. Note, however, that the 16-bit and 32-bit PCRE2 libraries process strings as sequences of uint16_t or uint32_t code points. They cannot find valid UTF sequences within an arbitrary string of bytes un- less such sequences are suitably aligned. This facility is not sup- ported for DFA matching. For details, see the pcre2unicode documenta- tion.
PCRE2_MATCH_UNSET_BACKREF
If this option is set, a backreference to an unset capture group matches an empty string (by default this causes the current matching alternative to fail). A pattern such as (\1)(a) succeeds when this op- tion is set (assuming it can find an "a" in the subject), whereas it fails by default, for Perl compatibility. Setting this option makes PCRE2 behave more like ECMAscript (aka JavaScript).
PCRE2_MULTILINE
By default, for the purposes of matching "start of line" and "end of line", PCRE2 treats the subject string as consisting of a single line of characters, even if it actually contains newlines. The "start of line" metacharacter (^) matches only at the start of the string, and the "end of line" metacharacter ($) matches only at the end of the string, or before a terminating newline (except when PCRE2_DOLLAR_EN- DONLY is set). Note, however, that unless PCRE2_DOTALL is set, the "any character" metacharacter (.) does not match at a newline. This behav- iour (for ^, $, and dot) is the same as Perl.
When PCRE2_MULTILINE it is set, the "start of line" and "end of line" constructs match immediately following or immediately before internal newlines in the subject string, respectively, as well as at the very start and end. This is equivalent to Perl's /m option, and it can be changed within a pattern by a (?m) option setting. Note that the "start of line" metacharacter does not match after a newline at the end of the subject, for compatibility with Perl. However, you can change this by setting the PCRE2_ALT_CIRCUMFLEX option. If there are no newlines in a subject string, or no occurrences of ^ or $ in a pattern, setting PCRE2_MULTILINE has no effect.
PCRE2_NEVER_BACKSLASH_C
This option locks out the use of \C in the pattern that is being com- piled. This escape can cause unpredictable behaviour in UTF-8 or UTF-16 modes, because it may leave the current matching point in the middle of a multi-code-unit character. This option may be useful in ap- plications that process patterns from external sources. Note that there is also a build-time option that permanently locks out the use of \C.
PCRE2_NEVER_UCP
This option locks out the use of Unicode properties for handling \B, \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes, as described for the PCRE2_UCP option below. In particular, it prevents the creator of the pattern from enabling this facility by starting the pattern with (*UCP). This option may be useful in applications that process patterns from external sources. The option combination PCRE_UCP and PCRE_NEVER_UCP causes an error.
PCRE2_NEVER_UTF
This option locks out interpretation of the pattern as UTF-8, UTF-16, or UTF-32, depending on which library is in use. In particular, it pre- vents the creator of the pattern from switching to UTF interpretation by starting the pattern with (*UTF). This option may be useful in ap- plications that process patterns from external sources. The combination of PCRE2_UTF and PCRE2_NEVER_UTF causes an error.
PCRE2_NO_AUTO_CAPTURE
If this option is set, it disables the use of numbered capturing paren- theses in the pattern. Any opening parenthesis that is not followed by ? behaves as if it were followed by ?: but named parentheses can still be used for capturing (and they acquire numbers in the usual way). This is the same as Perl's /n option. Note that, when this option is set, references to capture groups (backreferences or recursion/subroutine calls) may only refer to named groups, though the reference can be by name or by number.
PCRE2_NO_AUTO_POSSESS
If this option is set, it disables "auto-possessification", which is an optimization that, for example, turns a+b into a++b in order to avoid backtracks into a+ that can never be successful. However, if callouts are in use, auto-possessification means that some callouts are never taken. You can set this option if you want the matching functions to do a full unoptimized search and run all the callouts, but it is mainly provided for testing purposes.
PCRE2_NO_DOTSTAR_ANCHOR
If this option is set, it disables an optimization that is applied when .* is the first significant item in a top-level branch of a pattern, and all the other branches also start with .* or with \A or \G or ^. The optimization is automatically disabled for .* if it is inside an atomic group or a capture group that is the subject of a backreference, or if the pattern contains (*PRUNE) or (*SKIP). When the optimization is not disabled, such a pattern is automatically anchored if PCRE2_DOTALL is set for all the .* items and PCRE2_MULTILINE is not set for any ^ items. Otherwise, the fact that any match must start either at the start of the subject or following a newline is remembered. Like other optimizations, this can cause callouts to be skipped.
PCRE2_NO_START_OPTIMIZE
This is an option whose main effect is at matching time. It does not change what pcre2_compile() generates, but it does affect the output of the JIT compiler.
There are a number of optimizations that may occur at the start of a match, in order to speed up the process. For example, if it is known that an unanchored match must start with a specific code unit value, the matching code searches the subject for that value, and fails imme- diately if it cannot find it, without actually running the main match- ing function. This means that a special item such as (*COMMIT) at the start of a pattern is not considered until after a suitable starting point for the match has been found. Also, when callouts or (*MARK) items are in use, these "start-up" optimizations can cause them to be skipped if the pattern is never actually used. The start-up optimiza- tions are in effect a pre-scan of the subject that takes place before the pattern is run.
The PCRE2_NO_START_OPTIMIZE option disables the start-up optimizations, possibly causing performance to suffer, but ensuring that in cases where the result is "no match", the callouts do occur, and that items such as (*COMMIT) and (*MARK) are considered at every possible starting position in the subject string.
Setting PCRE2_NO_START_OPTIMIZE may change the outcome of a matching operation. Consider the pattern
(*COMMIT)ABC
When this is compiled, PCRE2 records the fact that a match must start with the character "A". Suppose the subject string is "DEFABC". The start-up optimization scans along the subject, finds "A" and runs the first match attempt from there. The (*COMMIT) item means that the pat- tern must match the current starting position, which in this case, it does. However, if the same match is run with PCRE2_NO_START_OPTIMIZE set, the initial scan along the subject string does not happen. The first match attempt is run starting from "D" and when this fails, (*COMMIT) prevents any further matches being tried, so the overall re- sult is "no match".
As another start-up optimization makes use of a minimum length for a matching subject, which is recorded when possible. Consider the pattern
(*MARK:1)B(*MARK:2)(X|Y)
The minimum length for a match is two characters. If the subject is "XXBB", the "starting character" optimization skips "XX", then tries to match "BB", which is long enough. In the process, (*MARK:2) is encoun- tered and remembered. When the match attempt fails, the next "B" is found, but there is only one character left, so there are no more at- tempts, and "no match" is returned with the "last mark seen" set to "2". If NO_START_OPTIMIZE is set, however, matches are tried at every possible starting position, including at the end of the subject, where (*MARK:1) is encountered, but there is no "B", so the "last mark seen" that is returned is "1". In this case, the optimizations do not affect the overall match result, which is still "no match", but they do affect the auxiliary information that is returned.
PCRE2_NO_UTF_CHECK
When PCRE2_UTF is set, the validity of the pattern as a UTF string is automatically checked. There are discussions about the validity of UTF-8 strings, UTF-16 strings, and UTF-32 strings in the pcre2unicode document. If an invalid UTF sequence is found, pcre2_compile() returns a negative error code.
If you know that your pattern is a valid UTF string, and you want to skip this check for performance reasons, you can set the PCRE2_NO_UTF_CHECK option. When it is set, the effect of passing an in- valid UTF string as a pattern is undefined. It may cause your program to crash or loop.
Note that this option can also be passed to pcre2_match() and pcre2_dfa_match(), to suppress UTF validity checking of the subject string.
Note also that setting PCRE2_NO_UTF_CHECK at compile time does not dis- able the error that is given if an escape sequence for an invalid Uni- code code point is encountered in the pattern. In particular, the so- called "surrogate" code points (0xd800 to 0xdfff) are invalid. If you want to allow escape sequences such as \x{d800} you can set the PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES extra option, as described in the section entitled "Extra compile options" below. However, this is pos- sible only in UTF-8 and UTF-32 modes, because these values are not rep- resentable in UTF-16.
PCRE2_UCP
This option has two effects. Firstly, it change the way PCRE2 processes \B, \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes. By default, only ASCII characters are recognized, but if PCRE2_UCP is set, Unicode properties are used to classify characters. There are some PCRE2_EXTRA options (see below) that add finer control to this behaviour. More details are given in the section on generic character types in the pcre2pattern page.
The second effect of PCRE2_UCP is to force the use of Unicode proper- ties for upper/lower casing operations, even when PCRE2_UTF is not set. This makes it possible to process strings in the 16-bit UCS-2 code. This option is available only if PCRE2 has been compiled with Unicode support (which is the default). The PCRE2_EXTRA_CASELESS_RESTRICT op- tion (see below) restricts caseless matching such that ASCII characters match only ASCII characters and non-ASCII characters match only non- ASCII characters.
PCRE2_UNGREEDY
This option inverts the "greediness" of the quantifiers so that they are not greedy by default, but become greedy if followed by "?". It is not compatible with Perl. It can also be set by a (?U) option setting within the pattern.
PCRE2_USE_OFFSET_LIMIT
This option must be set for pcre2_compile() if pcre2_set_offset_limit() is going to be used to set a non-default offset limit in a match con- text for matches that use this pattern. An error is generated if an offset limit is set without this option. For more details, see the de- scription of pcre2_set_offset_limit() in the section that describes match contexts. See also the PCRE2_FIRSTLINE option above.
PCRE2_UTF
This option causes PCRE2 to regard both the pattern and the subject strings that are subsequently processed as strings of UTF characters instead of single-code-unit strings. It is available when PCRE2 is built to include Unicode support (which is the default). If Unicode support is not available, the use of this option provokes an error. De- tails of how PCRE2_UTF changes the behaviour of PCRE2 are given in the pcre2unicode page. In particular, note that it changes the way PCRE2_CASELESS works.
Extra compile options
The option bits that can be set in a compile context by calling the pcre2_set_compile_extra_options() function are as follows:
PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK
Since release 10.38 PCRE2 has forbidden the use of \K within lookaround assertions, following Perl's lead. This option is provided to re-enable the previous behaviour (act in positive lookarounds, ignore in negative ones) in case anybody is relying on it.
PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES
This option applies when compiling a pattern in UTF-8 or UTF-32 mode. It is forbidden in UTF-16 mode, and ignored in non-UTF modes. Unicode "surrogate" code points in the range 0xd800 to 0xdfff are used in pairs in UTF-16 to encode code points with values in the range 0x10000 to 0x10ffff. The surrogates cannot therefore be represented in UTF-16. They can be represented in UTF-8 and UTF-32, but are defined as invalid code points, and cause errors if encountered in a UTF-8 or UTF-32 string that is being checked for validity by PCRE2.
These values also cause errors if encountered in escape sequences such as \x{d912} within a pattern. However, it seems that some applications, when using PCRE2 to check for unwanted characters in UTF-8 strings, ex- plicitly test for the surrogates using escape sequences. The PCRE2_NO_UTF_CHECK option does not disable the error that occurs, be- cause it applies only to the testing of input strings for UTF validity.
If the extra option PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES is set, surro- gate code point values in UTF-8 and UTF-32 patterns no longer provoke errors and are incorporated in the compiled pattern. However, they can only match subject characters if the matching function is called with PCRE2_NO_UTF_CHECK set.
PCRE2_EXTRA_ALT_BSUX
The original option PCRE2_ALT_BSUX causes PCRE2 to process \U, \u, and \x in the way that ECMAscript (aka JavaScript) does. Additional func- tionality was defined by ECMAscript 6; setting PCRE2_EXTRA_ALT_BSUX has the effect of PCRE2_ALT_BSUX, but in addition it recognizes \u{hhh..} as a hexadecimal character code, where hhh.. is any number of hexadeci- mal digits.
PCRE2_EXTRA_ASCII_BSD
This option forces \d to match only ASCII digits, even when PCRE2_UCP is set. It can be changed within a pattern by means of the (?aD) op- tion setting.
PCRE2_EXTRA_ASCII_BSS
This option forces \s to match only ASCII space characters, even when PCRE2_UCP is set. It can be changed within a pattern by means of the (?aS) option setting.
PCRE2_EXTRA_ASCII_BSW
This option forces \w to match only ASCII word characters, even when PCRE2_UCP is set. It can be changed within a pattern by means of the (?aW) option setting.
PCRE2_EXTRA_ASCII_DIGIT
This option forces the POSIX character classes [:digit:] and [:xdigit:] to match only ASCII digits, even when PCRE2_UCP is set. It can be changed within a pattern by means of the (?aT) option setting.
PCRE2_EXTRA_ASCII_POSIX
This option forces all the POSIX character classes, including [:digit:] and [:xdigit:], to match only ASCII characters, even when PCRE2_UCP is set. It can be changed within a pattern by means of the (?aP) option setting, but note that this also sets PCRE2_EXTRA_ASCII_DIGIT in order to ensure that (?-aP) unsets all ASCII restrictions for POSIX classes.
PCRE2_EXTRA_BAD_ESCAPE_IS_LITERAL
This is a dangerous option. Use with care. By default, an unrecognized escape such as \j or a malformed one such as \x{2z} causes a compile- time error when detected by pcre2_compile(). Perl is somewhat inconsis- tent in handling such items: for example, \j is treated as a literal "j", and non-hexadecimal digits in \x{} are just ignored, though warn- ings are given in both cases if Perl's warning switch is enabled. How- ever, a malformed octal number after \o{ always causes an error in Perl.
If the PCRE2_EXTRA_BAD_ESCAPE_IS_LITERAL extra option is passed to pcre2_compile(), all unrecognized or malformed escape sequences are treated as single-character escapes. For example, \j is a literal "j" and \x{2z} is treated as the literal string "x{2z}". Setting this op- tion means that typos in patterns may go undetected and have unexpected results. Also note that a sequence such as [\N{] is interpreted as a malformed attempt at [\N{...}] and so is treated as [N{] whereas [\N] gives an error because an unqualified \N is a valid escape sequence but is not supported in a character class. To reiterate: this is a danger- ous option. Use with great care.
PCRE2_EXTRA_CASELESS_RESTRICT
When either PCRE2_UCP or PCRE2_UTF is set, caseless matching follows Unicode rules, which allow for more than two cases per character. There are two case-equivalent character sets that contain both ASCII and non- ASCII characters. The ASCII letter S is case-equivalent to U+017f (long S) and the ASCII letter K is case-equivalent to U+212a (Kelvin sign). This option disables recognition of case-equivalences that cross the ASCII/non-ASCII boundary. In a caseless match, both characters must ei- ther be ASCII or non-ASCII. The option can be changed with a pattern by the (?r) option setting.
PCRE2_EXTRA_ESCAPED_CR_IS_LF
There are some legacy applications where the escape sequence \r in a pattern is expected to match a newline. If this option is set, \r in a pattern is converted to \n so that it matches a LF (linefeed) instead of a CR (carriage return) character. The option does not affect a lit- eral CR in the pattern, nor does it affect CR specified as an explicit code point such as \x{0D}.
PCRE2_EXTRA_MATCH_LINE
This option is provided for use by the -x option of pcre2grep. It causes the pattern only to match complete lines. This is achieved by automatically inserting the code for "^(?:" at the start of the com- piled pattern and ")$" at the end. Thus, when PCRE2_MULTILINE is set, the matched line may be in the middle of the subject string. This op- tion can be used with PCRE2_LITERAL.
PCRE2_EXTRA_MATCH_WORD
This option is provided for use by the -w option of pcre2grep. It causes the pattern only to match strings that have a word boundary at the start and the end. This is achieved by automatically inserting the code for "\b(?:" at the start of the compiled pattern and ")\b" at the end. The option may be used with PCRE2_LITERAL. However, it is ignored if PCRE2_EXTRA_MATCH_LINE is also set.
JUST-IN-TIME (JIT) COMPILATION
int pcre2_jit_compile(pcre2_code *code, uint32_t options);
int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext);
void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);
pcre2_jit_stack *pcre2_jit_stack_create(size_t startsize, size_t maxsize, pcre2_general_context *gcontext);
void pcre2_jit_stack_assign(pcre2_match_context *mcontext, pcre2_jit_callback callback_function, void *callback_data);
void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack);
These functions provide support for JIT compilation, which, if the just-in-time compiler is available, further processes a compiled pat- tern into machine code that executes much faster than the pcre2_match() interpretive matching function. Full details are given in the pcre2jit documentation.
JIT compilation is a heavyweight optimization. It can take some time for patterns to be analyzed, and for one-off matches and simple pat- terns the benefit of faster execution might be offset by a much slower compilation time. Most (but not all) patterns can be optimized by the JIT compiler.
LOCALE SUPPORT
const uint8_t *pcre2_maketables(pcre2_general_context *gcontext);
void pcre2_maketables_free(pcre2_general_context *gcontext, const uint8_t *tables);
PCRE2 handles caseless matching, and determines whether characters are letters, digits, or whatever, by reference to a set of tables, indexed by character code point. However, this applies only to characters whose code points are less than 256. By default, higher-valued code points never match escapes such as \w or \d.
When PCRE2 is built with Unicode support (the default), certain Unicode character properties can be tested with \p and \P, or, alternatively, the PCRE2_UCP option can be set when a pattern is compiled; this causes \w and friends to use Unicode property support instead of the built-in tables. PCRE2_UCP also causes upper/lower casing operations on charac- ters with code points greater than 127 to use Unicode properties. These effects apply even when PCRE2_UTF is not set. There are, however, some PCRE2_EXTRA options (see above) that can be used to modify or suppress them.
The use of locales with Unicode is discouraged. If you are handling characters with code points greater than 127, you should either use Unicode support, or use locales, but not try to mix the two.
PCRE2 contains a built-in set of character tables that are used by de- fault. These are sufficient for many applications. Normally, the in- ternal tables recognize only ASCII characters. However, when PCRE2 is built, it is possible to cause the internal tables to be rebuilt in the default "C" locale of the local system, which may cause them to be dif- ferent.
The built-in tables can be overridden by tables supplied by the appli- cation that calls PCRE2. These may be created in a different locale from the default. As more and more applications change to using Uni- code, the need for this locale support is expected to die away.
External tables are built by calling the pcre2_maketables() function, in the relevant locale. The only argument to this function is a general context, which can be used to pass a custom memory allocator. If the argument is NULL, the system malloc() is used. The result can be passed to pcre2_compile() as often as necessary, by creating a compile context and calling pcre2_set_character_tables() to set the tables pointer therein.
For example, to build and use tables that are appropriate for the French locale (where accented characters with values greater than 127 are treated as letters), the following code could be used:
setlocale(LC_CTYPE, "fr_FR"); tables = pcre2_maketables(NULL); ccontext = pcre2_compile_context_create(NULL); pcre2_set_character_tables(ccontext, tables); re = pcre2_compile(..., ccontext);
The locale name "fr_FR" is used on Linux and other Unix-like systems; if you are using Windows, the name for the French locale is "french".
The pointer that is passed (via the compile context) to pcre2_compile() is saved with the compiled pattern, and the same tables are used by the matching functions. Thus, for any single pattern, compilation and matching both happen in the same locale, but different patterns can be processed in different locales.
It is the caller's responsibility to ensure that the memory containing the tables remains available while they are still in use. When they are no longer needed, you can discard them using pcre2_maketables_free(), which should pass as its first parameter the same global context that was used to create the tables.
Saving locale tables
The tables described above are just a sequence of binary bytes, which makes them independent of hardware characteristics such as endianness or whether the processor is 32-bit or 64-bit. A copy of the result of pcre2_maketables() can therefore be saved in a file or elsewhere and re-used later, even in a different program or on another computer. The size of the tables (number of bytes) must be obtained by calling pcre2_config() with the PCRE2_CONFIG_TABLES_LENGTH option because pcre2_maketables() does not return this value. Note that the pcre2_dftables program, which is part of the PCRE2 build system, can be used stand-alone to create a file that contains a set of binary tables. See the pcre2build documentation for details.
INFORMATION ABOUT A COMPILED PATTERN
int pcre2_pattern_info(const pcre2 *code, uint32_t what, void *where);
The pcre2_pattern_info() function returns general information about a compiled pattern. For information about callouts, see the next section. The first argument for pcre2_pattern_info() is a pointer to the com- piled pattern. The second argument specifies which piece of information is required, and the third argument is a pointer to a variable to re- ceive the data. If the third argument is NULL, the first argument is ignored, and the function returns the size in bytes of the variable that is required for the information requested. Otherwise, the yield of the function is zero for success, or one of the following negative num- bers:
PCRE2_ERROR_NULL the argument code was NULL PCRE2_ERROR_BADMAGIC the "magic number" was not found PCRE2_ERROR_BADOPTION the value of what was invalid PCRE2_ERROR_UNSET the requested field is not set
The "magic number" is placed at the start of each compiled pattern as a simple check against passing an arbitrary memory pointer. Here is a typical call of pcre2_pattern_info(), to obtain the length of the com- piled pattern:
int rc; size_t length; rc = pcre2_pattern_info( re, /* result of pcre2_compile() */ PCRE2_INFO_SIZE, /* what is required */ &length); /* where to put the data */
The possible values for the second argument are defined in pcre2.h, and are as follows:
PCRE2_INFO_ALLOPTIONS PCRE2_INFO_ARGOPTIONS PCRE2_INFO_EXTRAOPTIONS
Return copies of the pattern's options. The third argument should point to a uint32_t variable. PCRE2_INFO_ARGOPTIONS returns exactly the op- tions that were passed to pcre2_compile(), whereas PCRE2_INFO_ALLOP- TIONS returns the compile options as modified by any top-level (*XXX) option settings such as (*UTF) at the start of the pattern itself. PCRE2_INFO_EXTRAOPTIONS returns the extra options that were set in the compile context by calling the pcre2_set_compile_extra_options() func- tion.
For example, if the pattern /(*UTF)abc/ is compiled with the PCRE2_EX- TENDED option, the result for PCRE2_INFO_ALLOPTIONS is PCRE2_EXTENDED and PCRE2_UTF. Option settings such as (?i) that can change within a pattern do not affect the result of PCRE2_INFO_ALLOPTIONS, even if they appear right at the start of the pattern. (This was different in some earlier releases.)
A pattern compiled without PCRE2_ANCHORED is automatically anchored by PCRE2 if the first significant item in every top-level branch is one of the following:
^ unless PCRE2_MULTILINE is set \A always \G always .* sometimes - see below
When .* is the first significant item, anchoring is possible only when all the following are true:
.* is not in an atomic group .* is not in a capture group that is the subject of a backreference PCRE2_DOTALL is in force for .* Neither (*PRUNE) nor (*SKIP) appears in the pattern PCRE2_NO_DOTSTAR_ANCHOR is not set
For patterns that are auto-anchored, the PCRE2_ANCHORED bit is set in the options returned for PCRE2_INFO_ALLOPTIONS.
PCRE2_INFO_BACKREFMAX
Return the number of the highest backreference in the pattern. The third argument should point to a uint32_t variable. Named capture groups acquire numbers as well as names, and these count towards the highest backreference. Backreferences such as \4 or \g{12} match the captured characters of the given group, but in addition, the check that a capture group is set in a conditional group such as (?(3)a|b) is also a backreference. Zero is returned if there are no backreferences.
PCRE2_INFO_BSR
The output is a uint32_t integer whose value indicates what character sequences the \R escape sequence matches. A value of PCRE2_BSR_UNICODE means that \R matches any Unicode line ending sequence; a value of PCRE2_BSR_ANYCRLF means that \R matches only CR, LF, or CRLF.
PCRE2_INFO_CAPTURECOUNT
Return the highest capture group number in the pattern. In patterns where (?| is not used, this is also the total number of capture groups. The third argument should point to a uint32_t variable.
PCRE2_INFO_DEPTHLIMIT
If the pattern set a backtracking depth limit by including an item of the form (*LIMIT_DEPTH=nnnn) at the start, the value is returned. The third argument should point to a uint32_t integer. If no such value has been set, the call to pcre2_pattern_info() returns the error PCRE2_ER- ROR_UNSET. Note that this limit will only be used during matching if it is less than the limit set or defaulted by the caller of the match function.
PCRE2_INFO_FIRSTBITMAP
In the absence of a single first code unit for a non-anchored pattern, pcre2_compile() may construct a 256-bit table that defines a fixed set of values for the first code unit in any match. For example, a pattern that starts with [abc] results in a table with three bits set. When code unit values greater than 255 are supported, the flag bit for 255 means "any code unit of value 255 or above". If such a table was con- structed, a pointer to it is returned. Otherwise NULL is returned. The third argument should point to a const uint8_t * variable.
PCRE2_INFO_FIRSTCODETYPE
Return information about the first code unit of any matched string, for a non-anchored pattern. The third argument should point to a uint32_t variable. If there is a fixed first value, for example, the letter "c" from a pattern such as (cat|cow|coyote), 1 is returned, and the value can be retrieved using PCRE2_INFO_FIRSTCODEUNIT. If there is no fixed first value, but it is known that a match can occur only at the start of the subject or following a newline in the subject, 2 is returned. Otherwise, and for anchored patterns, 0 is returned.
PCRE2_INFO_FIRSTCODEUNIT
Return the value of the first code unit of any matched string for a pattern where PCRE2_INFO_FIRSTCODETYPE returns 1; otherwise return 0. The third argument should point to a uint32_t variable. In the 8-bit library, the value is always less than 256. In the 16-bit library the value can be up to 0xffff. In the 32-bit library in UTF-32 mode the value can be up to 0x10ffff, and up to 0xffffffff when not using UTF-32 mode.
PCRE2_INFO_FRAMESIZE
Return the size (in bytes) of the data frames that are used to remember backtracking positions when the pattern is processed by pcre2_match() without the use of JIT. The third argument should point to a size_t variable. The frame size depends on the number of capturing parentheses in the pattern. Each additional capture group adds two PCRE2_SIZE vari- ables.
PCRE2_INFO_HASBACKSLASHC
Return 1 if the pattern contains any instances of \C, otherwise 0. The third argument should point to a uint32_t variable.
PCRE2_INFO_HASCRORLF
Return 1 if the pattern contains any explicit matches for CR or LF characters, otherwise 0. The third argument should point to a uint32_t variable. An explicit match is either a literal CR or LF character, or \r or \n or one of the equivalent hexadecimal or octal escape se- quences.
PCRE2_INFO_HEAPLIMIT
If the pattern set a heap memory limit by including an item of the form (*LIMIT_HEAP=nnnn) at the start, the value is returned. The third argu- ment should point to a uint32_t integer. If no such value has been set, the call to pcre2_pattern_info() returns the error PCRE2_ERROR_UNSET. Note that this limit will only be used during matching if it is less than the limit set or defaulted by the caller of the match function.
PCRE2_INFO_JCHANGED
Return 1 if the (?J) or (?-J) option setting is used in the pattern, otherwise 0. The third argument should point to a uint32_t variable. (?J) and (?-J) set and unset the local PCRE2_DUPNAMES option, respec- tively.
PCRE2_INFO_JITSIZE
If the compiled pattern was successfully processed by pcre2_jit_com- pile(), return the size of the JIT compiled code, otherwise return zero. The third argument should point to a size_t variable.
PCRE2_INFO_LASTCODETYPE
Returns 1 if there is a rightmost literal code unit that must exist in any matched string, other than at its start. The third argument should point to a uint32_t variable. If there is no such value, 0 is returned. When 1 is returned, the code unit value itself can be retrieved using PCRE2_INFO_LASTCODEUNIT. For anchored patterns, a last literal value is recorded only if it follows something of variable length. For example, for the pattern /^a\d+z\d+/ the returned value is 1 (with "z" returned from PCRE2_INFO_LASTCODEUNIT), but for /^a\dz\d/ the returned value is 0.
PCRE2_INFO_LASTCODEUNIT
Return the value of the rightmost literal code unit that must exist in any matched string, other than at its start, for a pattern where PCRE2_INFO_LASTCODETYPE returns 1. Otherwise, return 0. The third argu- ment should point to a uint32_t variable.
PCRE2_INFO_MATCHEMPTY
Return 1 if the pattern might match an empty string, otherwise 0. The third argument should point to a uint32_t variable. When a pattern con- tains recursive subroutine calls it is not always possible to determine whether or not it can match an empty string. PCRE2 takes a cautious ap- proach and returns 1 in such cases.
PCRE2_INFO_MATCHLIMIT
If the pattern set a match limit by including an item of the form (*LIMIT_MATCH=nnnn) at the start, the value is returned. The third ar- gument should point to a uint32_t integer. If no such value has been set, the call to pcre2_pattern_info() returns the error PCRE2_ERROR_UN- SET. Note that this limit will only be used during matching if it is less than the limit set or defaulted by the caller of the match func- tion.
PCRE2_INFO_MAXLOOKBEHIND
A lookbehind assertion moves back a certain number of characters (not code units) when it starts to process each of its branches. This re- quest returns the largest of these backward moves. The third argument should point to a uint32_t integer. The simple assertions \b and \B re- quire a one-character lookbehind and cause PCRE2_INFO_MAXLOOKBEHIND to return 1 in the absence of anything longer. \A also registers a one- character lookbehind, though it does not actually inspect the previous character.
Note that this information is useful for multi-segment matching only if the pattern contains no nested lookbehinds. For example, the pattern (?<=a(?<=ba)c) returns a maximum lookbehind of 2, but when it is processed, the first lookbehind moves back by two characters, matches one character, then the nested lookbehind also moves back by two char- acters. This puts the matching point three characters earlier than it was at the start. PCRE2_INFO_MAXLOOKBEHIND is really only useful as a debugging tool. See the pcre2partial documentation for a discussion of multi-segment matching.
PCRE2_INFO_MINLENGTH
If a minimum length for matching subject strings was computed, its value is returned. Otherwise the returned value is 0. This value is not computed when PCRE2_NO_START_OPTIMIZE is set. The value is a number of characters, which in UTF mode may be different from the number of code units. The third argument should point to a uint32_t variable. The value is a lower bound to the length of any matching string. There may not be any strings of that length that do actually match, but every string that does match is at least that long.
PCRE2_INFO_NAMECOUNT PCRE2_INFO_NAMEENTRYSIZE PCRE2_INFO_NAMETABLE
PCRE2 supports the use of named as well as numbered capturing parenthe- ses. The names are just an additional way of identifying the parenthe- ses, which still acquire numbers. Several convenience functions such as pcre2_substring_get_byname() are provided for extracting captured sub- strings by name. It is also possible to extract the data directly, by first converting the name to a number in order to access the correct pointers in the output vector (described with pcre2_match() below). To do the conversion, you need to use the name-to-number map, which is de- scribed by these three values.
The map consists of a number of fixed-size entries. PCRE2_INFO_NAME- COUNT gives the number of entries, and PCRE2_INFO_NAMEENTRYSIZE gives the size of each entry in code units; both of these return a uint32_t value. The entry size depends on the length of the longest name.
PCRE2_INFO_NAMETABLE returns a pointer to the first entry of the table. This is a PCRE2_SPTR pointer to a block of code units. In the 8-bit li- brary, the first two bytes of each entry are the number of the captur- ing parenthesis, most significant byte first. In the 16-bit library, the pointer points to 16-bit code units, the first of which contains the parenthesis number. In the 32-bit library, the pointer points to 32-bit code units, the first of which contains the parenthesis number. The rest of the entry is the corresponding name, zero terminated.
The names are in alphabetical order. If (?| is used to create multiple capture groups with the same number, as described in the section on du- plicate group numbers in the pcre2pattern page, the groups may be given the same name, but there is only one entry in the table. Different names for groups of the same number are not permitted.
Duplicate names for capture groups with different numbers are permit- ted, but only if PCRE2_DUPNAMES is set. They appear in the table in the order in which they were found in the pattern. In the absence of (?| this is the order of increasing number; when (?| is used this is not necessarily the case because later capture groups may have lower num- bers.
As a simple example of the name/number table, consider the following pattern after compilation by the 8-bit library (assume PCRE2_EXTENDED is set, so white space - including newlines - is ignored):
(?<date> (?<year>(\d\d)?\d\d) - (?<month>\d\d) - (?<day>\d\d) )
There are four named capture groups, so the table has four entries, and each entry in the table is eight bytes long. The table is as follows, with non-printing bytes shows in hexadecimal, and undefined bytes shown as ??:
00 01 d a t e 00 ?? 00 05 d a y 00 ?? ?? 00 04 m o n t h 00 00 02 y e a r 00 ??
When writing code to extract data from named capture groups using the name-to-number map, remember that the length of the entries is likely to be different for each compiled pattern.
PCRE2_INFO_NEWLINE
The output is one of the following uint32_t values:
PCRE2_NEWLINE_CR Carriage return (CR) PCRE2_NEWLINE_LF Linefeed (LF) PCRE2_NEWLINE_CRLF Carriage return, linefeed (CRLF) PCRE2_NEWLINE_ANY Any Unicode line ending PCRE2_NEWLINE_ANYCRLF Any of CR, LF, or CRLF PCRE2_NEWLINE_NUL The NUL character (binary zero)
This identifies the character sequence that will be recognized as mean- ing "newline" while matching.
PCRE2_INFO_SIZE
Return the size of the compiled pattern in bytes (for all three li- braries). The third argument should point to a size_t variable. This value includes the size of the general data block that precedes the code units of the compiled pattern itself. The value that is used when pcre2_compile() is getting memory in which to place the compiled pat- tern may be slightly larger than the value returned by this option, be- cause there are cases where the code that calculates the size has to over-estimate. Processing a pattern with the JIT compiler does not al- ter the value returned by this option.
INFORMATION ABOUT A PATTERN'S CALLOUTS
int pcre2_callout_enumerate(const pcre2_code *code, int (*callback)(pcre2_callout_enumerate_block *, void *), void *user_data);
A script language that supports the use of string arguments in callouts might like to scan all the callouts in a pattern before running the match. This can be done by calling pcre2_callout_enumerate(). The first argument is a pointer to a compiled pattern, the second points to a callback function, and the third is arbitrary user data. The callback function is called for every callout in the pattern in the order in which they appear. Its first argument is a pointer to a callout enumer- ation block, and its second argument is the user_data value that was passed to pcre2_callout_enumerate(). The contents of the callout enu- meration block are described in the pcre2callout documentation, which also gives further details about callouts.
SERIALIZATION AND PRECOMPILING
It is possible to save compiled patterns on disc or elsewhere, and re- load them later, subject to a number of restrictions. The host on which the patterns are reloaded must be running the same version of PCRE2, with the same code unit width, and must also have the same endianness, pointer width, and PCRE2_SIZE type. Before compiled patterns can be saved, they must be converted to a "serialized" form, which in the case of PCRE2 is really just a bytecode dump. The functions whose names be- gin with pcre2_serialize_ are used for converting to and from the seri- alized form. They are described in the pcre2serialize documentation. Note that PCRE2 serialization does not convert compiled patterns to an abstract format like Java or .NET serialization.
THE MATCH DATA BLOCK
pcre2_match_data *pcre2_match_data_create(uint32_t ovecsize, pcre2_general_context *gcontext);
pcre2_match_data *pcre2_match_data_create_from_pattern( const pcre2_code *code, pcre2_general_context *gcontext);
void pcre2_match_data_free(pcre2_match_data *match_data);
Information about a successful or unsuccessful match is placed in a match data block, which is an opaque structure that is accessed by function calls. In particular, the match data block contains a vector of offsets into the subject string that define the matched parts of the subject. This is known as the ovector.
Before calling pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match() you must create a match data block by calling one of the creation func- tions above. For pcre2_match_data_create(), the first argument is the number of pairs of offsets in the ovector.
When using pcre2_match(), one pair of offsets is required to identify the string that matched the whole pattern, with an additional pair for each captured substring. For example, a value of 4 creates enough space to record the matched portion of the subject plus three captured sub- strings.
When using pcre2_dfa_match() there may be multiple matched substrings of different lengths at the same point in the subject. The ovector should be made large enough to hold as many as are expected.
A minimum of at least 1 pair is imposed by pcre2_match_data_create(), so it is always possible to return the overall matched string in the case of pcre2_match() or the longest match in the case of pcre2_dfa_match(). The maximum number of pairs is 65535; if the first argument of pcre2_match_data_create() is greater than this, 65535 is used.
The second argument of pcre2_match_data_create() is a pointer to a gen- eral context, which can specify custom memory management for obtaining the memory for the match data block. If you are not using custom memory management, pass NULL, which causes malloc() to be used.
For pcre2_match_data_create_from_pattern(), the first argument is a pointer to a compiled pattern. The ovector is created to be exactly the right size to hold all the substrings a pattern might capture when matched using pcre2_match(). You should not use this call when matching with pcre2_dfa_match(). The second argument is again a pointer to a general context, but in this case if NULL is passed, the memory is ob- tained using the same allocator that was used for the compiled pattern (custom or default).
A match data block can be used many times, with the same or different compiled patterns. You can extract information from a match data block after a match operation has finished, using functions that are de- scribed in the sections on matched strings and other match data below.
When a call of pcre2_match() fails, valid data is available in the match block only when the error is PCRE2_ERROR_NOMATCH, PCRE2_ER- ROR_PARTIAL, or one of the error codes for an invalid UTF string. Ex- actly what is available depends on the error, and is detailed below.
When one of the matching functions is called, pointers to the compiled pattern and the subject string are set in the match data block so that they can be referenced by the extraction functions after a successful match. After running a match, you must not free a compiled pattern or a subject string until after all operations on the match data block (for that match) have taken place, unless, in the case of the subject string, you have used the PCRE2_COPY_MATCHED_SUBJECT option, which is described in the section entitled "Option bits for pcre2_match()" be- low.
When a match data block itself is no longer needed, it should be freed by calling pcre2_match_data_free(). If this function is called with a NULL argument, it returns immediately, without doing anything.
MEMORY USE FOR MATCH DATA BLOCKS
PCRE2_SIZE pcre2_get_match_data_size(pcre2_match_data *match_data);
PCRE2_SIZE pcre2_get_match_data_heapframes_size( pcre2_match_data *match_data);
The size of a match data block depends on the size of the ovector that it contains. The function pcre2_get_match_data_size() returns the size, in bytes, of the block that is its argument.
When pcre2_match() runs interpretively (that is, without using JIT), it makes use of a vector of data frames for remembering backtracking posi- tions. The size of each individual frame depends on the number of cap- turing parentheses in the pattern and can be obtained by calling pcre2_pattern_info() with the PCRE2_INFO_FRAMESIZE option (see the sec- tion entitled "Information about a compiled pattern" above).
Heap memory is used for the frames vector; if the initial memory block turns out to be too small during matching, it is automatically ex- panded. When pcre2_match() returns, the memory is not freed, but re- mains attached to the match data block, for use by any subsequent matches that use the same block. It is automatically freed when the match data block itself is freed.
You can find the current size of the frames vector that a match data block owns by calling pcre2_get_match_data_heapframes_size(). For a newly created match data block the size will be zero. Some types of match may require a lot of frames and thus a large vector; applications that run in environments where memory is constrained can check this and free the match data block if the heap frames vector has become too big.
MATCHING A PATTERN: THE TRADITIONAL FUNCTION
int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext);
The function pcre2_match() is called to match a subject string against a compiled pattern, which is passed in the code argument. You can call pcre2_match() with the same code argument as many times as you like, in order to find multiple matches in the subject string or to match dif- ferent subject strings with the same pattern.
This function is the main matching facility of the library, and it op- erates in a Perl-like manner. For specialist use there is also an al- ternative matching function, which is described below in the section about the pcre2_dfa_match() function.
Here is an example of a simple call to pcre2_match():
pcre2_match_data *md = pcre2_match_data_create(4, NULL); int rc = pcre2_match( re, /* result of pcre2_compile() */ "some string", /* the subject string */ 11, /* the length of the subject string */ 0, /* start at offset 0 in the subject */ 0, /* default options */ md, /* the match data block */ NULL); /* a match context; NULL means use defaults */
If the subject string is zero-terminated, the length can be given as PCRE2_ZERO_TERMINATED. A match context must be provided if certain less common matching parameters are to be changed. For details, see the sec- tion on the match context above.
The string to be matched by pcre2_match()
The subject string is passed to pcre2_match() as a pointer in subject, a length in length, and a starting offset in startoffset. The length and offset are in code units, not characters. That is, they are in bytes for the 8-bit library, 16-bit code units for the 16-bit library, and 32-bit code units for the 32-bit library, whether or not UTF pro- cessing is enabled. As a special case, if subject is NULL and length is zero, the subject is assumed to be an empty string. If length is non- zero, an error occurs if subject is NULL.
If startoffset is greater than the length of the subject, pcre2_match() returns PCRE2_ERROR_BADOFFSET. When the starting offset is zero, the search for a match starts at the beginning of the subject, and this is by far the most common case. In UTF-8 or UTF-16 mode, the starting off- set must point to the start of a character, or to the end of the sub- ject (in UTF-32 mode, one code unit equals one character, so all off- sets are valid). Like the pattern string, the subject may contain bi- nary zeros.
A non-zero starting offset is useful when searching for another match in the same subject by calling pcre2_match() again after a previous success. Setting startoffset differs from passing over a shortened string and setting PCRE2_NOTBOL in the case of a pattern that begins with any kind of lookbehind. For example, consider the pattern
\Biss\B
which finds occurrences of "iss" in the middle of words. (\B matches only if the current position in the subject is not a word boundary.) When applied to the string "Mississippi" the first call to pcre2_match() finds the first occurrence. If pcre2_match() is called again with just the remainder of the subject, namely "issippi", it does not match, because \B is always false at the start of the subject, which is deemed to be a word boundary. However, if pcre2_match() is passed the entire string again, but with startoffset set to 4, it finds the second occurrence of "iss" because it is able to look behind the starting point to discover that it is preceded by a letter.
Finding all the matches in a subject is tricky when the pattern can match an empty string. It is possible to emulate Perl's /g behaviour by first trying the match again at the same offset, with the PCRE2_NOTEMPTY_ATSTART and PCRE2_ANCHORED options, and then if that fails, advancing the starting offset and trying an ordinary match again. There is some code that demonstrates how to do this in the pcre2demo sample program. In the most general case, you have to check to see if the newline convention recognizes CRLF as a newline, and if so, and the current character is CR followed by LF, advance the start- ing offset by two characters instead of one.
If a non-zero starting offset is passed when the pattern is anchored, a single attempt to match at the given offset is made. This can only suc- ceed if the pattern does not require the match to be at the start of the subject. In other words, the anchoring must be the result of set- ting the PCRE2_ANCHORED option or the use of .* with PCRE2_DOTALL, not by starting the pattern with ^ or \A.
Option bits for pcre2_match()
The unused bits of the options argument for pcre2_match() must be zero. The only bits that may be set are PCRE2_ANCHORED, PCRE2_COPY_MATCHED_SUBJECT, PCRE2_DISABLE_RECURSELOOP_CHECK, PCRE2_EN- DANCHORED, PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_JIT, PCRE2_NO_UTF_CHECK, PCRE2_PAR- TIAL_HARD, and PCRE2_PARTIAL_SOFT. Their action is described below.
Setting PCRE2_ANCHORED or PCRE2_ENDANCHORED at match time is not sup- ported by the just-in-time (JIT) compiler. If it is set, JIT matching is disabled and the interpretive code in pcre2_match() is run. PCRE2_DISABLE_RECURSELOOP_CHECK is ignored by JIT, but apart from PCRE2_NO_JIT (obviously), the remaining options are supported for JIT matching.
PCRE2_ANCHORED
The PCRE2_ANCHORED option limits pcre2_match() to matching at the first matching position. If a pattern was compiled with PCRE2_ANCHORED, or turned out to be anchored by virtue of its contents, it cannot be made unachored at matching time. Note that setting the option at match time disables JIT matching.
PCRE2_COPY_MATCHED_SUBJECT
By default, a pointer to the subject is remembered in the match data block so that, after a successful match, it can be referenced by the substring extraction functions. This means that the subject's memory must not be freed until all such operations are complete. For some ap- plications where the lifetime of the subject string is not guaranteed, it may be necessary to make a copy of the subject string, but it is wasteful to do this unless the match is successful. After a successful match, if PCRE2_COPY_MATCHED_SUBJECT is set, the subject is copied and the new pointer is remembered in the match data block instead of the original subject pointer. The memory allocator that was used for the match block itself is used. The copy is automatically freed when pcre2_match_data_free() is called to free the match data block. It is also automatically freed if the match data block is re-used for another match operation.
PCRE2_DISABLE_RECURSELOOP_CHECK
This option is relevant only to pcre2_match() for interpretive match- ing. It is ignored when JIT is used, and is forbidden for pcre2_dfa_match().
The use of recursion in patterns can lead to infinite loops. In the in- terpretive matcher these would be eventually caught by the match or heap limits, but this could take a long time and/or use a lot of memory if the limits are large. There is therefore a check at the start of each recursion. If the same group is still active from a previous call, and the current subject pointer is the same as it was at the start of that group, and the furthest inspected character of the sub- ject has not changed, an error is generated.
There are rare cases of matches that would complete, but nevertheless trigger this error. This option disables the check. It is provided mainly for testing when comparing JIT and interpretive behaviour.
PCRE2_ENDANCHORED
If the PCRE2_ENDANCHORED option is set, any string that pcre2_match() matches must be right at the end of the subject string. Note that set- ting the option at match time disables JIT matching.
PCRE2_NOTBOL
This option specifies that first character of the subject string is not the beginning of a line, so the circumflex metacharacter should not match before it. Setting this without having set PCRE2_MULTILINE at compile time causes circumflex never to match. This option affects only the behaviour of the circumflex metacharacter. It does not affect \A.
PCRE2_NOTEOL
This option specifies that the end of the subject string is not the end of a line, so the dollar metacharacter should not match it nor (except in multiline mode) a newline immediately before it. Setting this with- out having set PCRE2_MULTILINE at compile time causes dollar never to match. This option affects only the behaviour of the dollar metacharac- ter. It does not affect \Z or \z.
PCRE2_NOTEMPTY
An empty string is not considered to be a valid match if this option is set. If there are alternatives in the pattern, they are tried. If all the alternatives match the empty string, the entire match fails. For example, if the pattern
a?b?
is applied to a string not beginning with "a" or "b", it matches an empty string at the start of the subject. With PCRE2_NOTEMPTY set, this match is not valid, so pcre2_match() searches further into the string for occurrences of "a" or "b".
PCRE2_NOTEMPTY_ATSTART
This is like PCRE2_NOTEMPTY, except that it locks out an empty string match only at the first matching position, that is, at the start of the subject plus the starting offset. An empty string match later in the subject is permitted. If the pattern is anchored, such a match can oc- cur only if the pattern contains \K.
PCRE2_NO_JIT
By default, if a pattern has been successfully processed by pcre2_jit_compile(), JIT is automatically used when pcre2_match() is called with options that JIT supports. Setting PCRE2_NO_JIT disables the use of JIT; it forces matching to be done by the interpreter.
PCRE2_NO_UTF_CHECK
When PCRE2_UTF is set at compile time, the validity of the subject as a UTF string is checked unless PCRE2_NO_UTF_CHECK is passed to pcre2_match() or PCRE2_MATCH_INVALID_UTF was passed to pcre2_compile(). The latter special case is discussed in detail in the pcre2unicode doc- umentation.
In the default case, if a non-zero starting offset is given, the check is applied only to that part of the subject that could be inspected during matching, and there is a check that the starting offset points to the first code unit of a character or to the end of the subject. If there are no lookbehind assertions in the pattern, the check starts at the starting offset. Otherwise, it starts at the length of the longest lookbehind before the starting offset, or at the start of the subject if there are not that many characters before the starting offset. Note that the sequences \b and \B are one-character lookbehinds.
The check is carried out before any other processing takes place, and a negative error code is returned if the check fails. There are several UTF error codes for each code unit width, corresponding to different problems with the code unit sequence. There are discussions about the validity of UTF-8 strings, UTF-16 strings, and UTF-32 strings in the pcre2unicode documentation.
If you know that your subject is valid, and you want to skip this check for performance reasons, you can set the PCRE2_NO_UTF_CHECK option when calling pcre2_match(). You might want to do this for the second and subsequent calls to pcre2_match() if you are making repeated calls to find multiple matches in the same subject string.
Warning: Unless PCRE2_MATCH_INVALID_UTF was set at compile time, when PCRE2_NO_UTF_CHECK is set at match time the effect of passing an in- valid string as a subject, or an invalid value of startoffset, is unde- fined. Your program may crash or loop indefinitely or give wrong re- sults.
PCRE2_PARTIAL_HARD PCRE2_PARTIAL_SOFT
These options turn on the partial matching feature. A partial match oc- curs if the end of the subject string is reached successfully, but there are not enough subject characters to complete the match. In addi- tion, either at least one character must have been inspected or the pattern must contain a lookbehind, or the pattern must be one that could match an empty string.
If this situation arises when PCRE2_PARTIAL_SOFT (but not PCRE2_PAR- TIAL_HARD) is set, matching continues by testing any remaining alterna- tives. Only if no complete match can be found is PCRE2_ERROR_PARTIAL returned instead of PCRE2_ERROR_NOMATCH. In other words, PCRE2_PAR- TIAL_SOFT specifies that the caller is prepared to handle a partial match, but only if no complete match can be found.
If PCRE2_PARTIAL_HARD is set, it overrides PCRE2_PARTIAL_SOFT. In this case, if a partial match is found, pcre2_match() immediately returns PCRE2_ERROR_PARTIAL, without considering any other alternatives. In other words, when PCRE2_PARTIAL_HARD is set, a partial match is consid- ered to be more important that an alternative complete match.
There is a more detailed discussion of partial and multi-segment match- ing, with examples, in the pcre2partial documentation.
NEWLINE HANDLING WHEN MATCHING
When PCRE2 is built, a default newline convention is set; this is usu- ally the standard convention for the operating system. The default can be overridden in a compile context by calling pcre2_set_newline(). It can also be overridden by starting a pattern string with, for example, (*CRLF), as described in the section on newline conventions in the pcre2pattern page. During matching, the newline choice affects the be- haviour of the dot, circumflex, and dollar metacharacters. It may also alter the way the match starting position is advanced after a match failure for an unanchored pattern.
When PCRE2_NEWLINE_CRLF, PCRE2_NEWLINE_ANYCRLF, or PCRE2_NEWLINE_ANY is set as the newline convention, and a match attempt for an unanchored pattern fails when the current starting position is at a CRLF sequence, and the pattern contains no explicit matches for CR or LF characters, the match position is advanced by two characters instead of one, in other words, to after the CRLF.
The above rule is a compromise that makes the most common cases work as expected. For example, if the pattern is .+A (and the PCRE2_DOTALL op- tion is not set), it does not match the string "\r\nA" because, after failing at the start, it skips both the CR and the LF before retrying. However, the pattern [\r\n]A does match that string, because it con- tains an explicit CR or LF reference, and so advances only by one char- acter after the first failure.
An explicit match for CR of LF is either a literal appearance of one of those characters in the pattern, or one of the \r or \n or equivalent octal or hexadecimal escape sequences. Implicit matches such as [^X] do not count, nor does \s, even though it includes CR and LF in the char- acters that it matches.
Notwithstanding the above, anomalous effects may still occur when CRLF is a valid newline sequence and explicit \r or \n escapes appear in the pattern.
HOW PCRE2_MATCH() RETURNS A STRING AND CAPTURED SUBSTRINGS
uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data);
PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data);
In general, a pattern matches a certain portion of the subject, and in addition, further substrings from the subject may be picked out by parenthesized parts of the pattern. Following the usage in Jeffrey Friedl's book, this is called "capturing" in what follows, and the phrase "capture group" (Perl terminology) is used for a fragment of a pattern that picks out a substring. PCRE2 supports several other kinds of parenthesized group that do not cause substrings to be captured. The pcre2_pattern_info() function can be used to find out how many capture groups there are in a compiled pattern.
You can use auxiliary functions for accessing captured substrings by number or by name, as described in sections below.
Alternatively, you can make direct use of the vector of PCRE2_SIZE val- ues, called the ovector, which contains the offsets of captured strings. It is part of the match data block. The function pcre2_get_ovector_pointer() returns the address of the ovector, and pcre2_get_ovector_count() returns the number of pairs of values it con- tains.
Within the ovector, the first in each pair of values is set to the off- set of the first code unit of a substring, and the second is set to the offset of the first code unit after the end of a substring. These val- ues are always code unit offsets, not character offsets. That is, they are byte offsets in the 8-bit library, 16-bit offsets in the 16-bit li- brary, and 32-bit offsets in the 32-bit library.
After a partial match (error return PCRE2_ERROR_PARTIAL), only the first pair of offsets (that is, ovector[0] and ovector[1]) are set. They identify the part of the subject that was partially matched. See the pcre2partial documentation for details of partial matching.
After a fully successful match, the first pair of offsets identifies the portion of the subject string that was matched by the entire pat- tern. The next pair is used for the first captured substring, and so on. The value returned by pcre2_match() is one more than the highest numbered pair that has been set. For example, if two substrings have been captured, the returned value is 3. If there are no captured sub- strings, the return value from a successful match is 1, indicating that just the first pair of offsets has been set.
If a pattern uses the \K escape sequence within a positive assertion, the reported start of a successful match can be greater than the end of the match. For example, if the pattern (?=ab\K) is matched against "ab", the start and end offset values for the match are 2 and 0.
If a capture group is matched repeatedly within a single match opera- tion, it is the last portion of the subject that it matched that is re- turned.
If the ovector is too small to hold all the captured substring offsets, as much as possible is filled in, and the function returns a value of zero. If captured substrings are not of interest, pcre2_match() may be called with a match data block whose ovector is of minimum length (that is, one pair).
It is possible for capture group number n+1 to match some part of the subject when group n has not been used at all. For example, if the string "abc" is matched against the pattern (a|(z))(bc) the return from the function is 4, and groups 1 and 3 are matched, but 2 is not. When this happens, both values in the offset pairs corresponding to unused groups are set to PCRE2_UNSET.
Offset values that correspond to unused groups at the end of the ex- pression are also set to PCRE2_UNSET. For example, if the string "abc" is matched against the pattern (abc)(x(yz)?)? groups 2 and 3 are not matched. The return from the function is 2, because the highest used capture group number is 1. The offsets for the second and third capture groups (assuming the vector is large enough, of course) are set to PCRE2_UNSET.
Elements in the ovector that do not correspond to capturing parentheses in the pattern are never changed. That is, if a pattern contains n cap- turing parentheses, no more than ovector[0] to ovector[2n+1] are set by pcre2_match(). The other elements retain whatever values they previ- ously had. After a failed match attempt, the contents of the ovector are unchanged.
OTHER INFORMATION ABOUT A MATCH
PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data);
PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data);
As well as the offsets in the ovector, other information about a match is retained in the match data block and can be retrieved by the above functions in appropriate circumstances. If they are called at other times, the result is undefined.
After a successful match, a partial match (PCRE2_ERROR_PARTIAL), or a failure to match (PCRE2_ERROR_NOMATCH), a mark name may be available. The function pcre2_get_mark() can be called to access this name, which can be specified in the pattern by any of the backtracking control verbs, not just (*MARK). The same function applies to all the verbs. It returns a pointer to the zero-terminated name, which is within the com- piled pattern. If no name is available, NULL is returned. The length of the name (excluding the terminating zero) is stored in the code unit that precedes the name. You should use this length instead of relying on the terminating zero if the name might contain a binary zero.
After a successful match, the name that is returned is the last mark name encountered on the matching path through the pattern. Instances of backtracking verbs without names do not count. Thus, for example, if the matching path contains (*MARK:A)(*PRUNE), the name "A" is returned. After a "no match" or a partial match, the last encountered name is re- turned. For example, consider this pattern:
^(*MARK:A)((*MARK:B)a|b)c
When it matches "bc", the returned name is A. The B mark is "seen" in the first branch of the group, but it is not on the matching path. On the other hand, when this pattern fails to match "bx", the returned name is B.
Warning: By default, certain start-of-match optimizations are used to give a fast "no match" result in some situations. For example, if the anchoring is removed from the pattern above, there is an initial check for the presence of "c" in the subject before running the matching en- gine. This check fails for "bx", causing a match failure without seeing any marks. You can disable the start-of-match optimizations by setting the PCRE2_NO_START_OPTIMIZE option for pcre2_compile() or by starting the pattern with (*NO_START_OPT).
After a successful match, a partial match, or one of the invalid UTF errors (for example, PCRE2_ERROR_UTF8_ERR5), pcre2_get_startchar() can be called. After a successful or partial match it returns the code unit offset of the character at which the match started. For a non-partial match, this can be different to the value of ovector[0] if the pattern contains the \K escape sequence. After a partial match, however, this value is always the same as ovector[0] because \K does not affect the result of a partial match.
After a UTF check failure, pcre2_get_startchar() can be used to obtain the code unit offset of the invalid UTF character. Details are given in the pcre2unicode page.
ERROR RETURNS FROM pcre2_match()
If pcre2_match() fails, it returns a negative number. This can be con- verted to a text string by calling the pcre2_get_error_message() func- tion (see "Obtaining a textual error message" below). Negative error codes are also returned by other functions, and are documented with them. The codes are given names in the header file. If UTF checking is in force and an invalid UTF subject string is detected, one of a number of UTF-specific negative error codes is returned. Details are given in the pcre2unicode page. The following are the other errors that may be returned by pcre2_match():
PCRE2_ERROR_NOMATCH
The subject string did not match the pattern.
PCRE2_ERROR_PARTIAL
The subject string did not match, but it did match partially. See the pcre2partial documentation for details of partial matching.
PCRE2_ERROR_BADMAGIC
PCRE2 stores a 4-byte "magic number" at the start of the compiled code, to catch the case when it is passed a junk pointer. This is the error that is returned when the magic number is not present.
PCRE2_ERROR_BADMODE
This error is given when a compiled pattern is passed to a function in a library of a different code unit width, for example, a pattern com- piled by the 8-bit library is passed to a 16-bit or 32-bit library function.
PCRE2_ERROR_BADOFFSET
The value of startoffset was greater than the length of the subject.
PCRE2_ERROR_BADOPTION
An unrecognized bit was set in the options argument.
PCRE2_ERROR_BADUTFOFFSET
The UTF code unit sequence that was passed as a subject was checked and found to be valid (the PCRE2_NO_UTF_CHECK option was not set), but the value of startoffset did not point to the beginning of a UTF character or the end of the subject.
PCRE2_ERROR_CALLOUT
This error is never generated by pcre2_match() itself. It is provided for use by callout functions that want to cause pcre2_match() or pcre2_callout_enumerate() to return a distinctive error code. See the pcre2callout documentation for details.
PCRE2_ERROR_DEPTHLIMIT
The nested backtracking depth limit was reached.
PCRE2_ERROR_HEAPLIMIT
The heap limit was reached.
PCRE2_ERROR_INTERNAL
An unexpected internal error has occurred. This error could be caused by a bug in PCRE2 or by overwriting of the compiled pattern.
PCRE2_ERROR_JIT_STACKLIMIT
This error is returned when a pattern that was successfully studied us- ing JIT is being matched, but the memory available for the just-in-time processing stack is not large enough. See the pcre2jit documentation for more details.
PCRE2_ERROR_MATCHLIMIT
The backtracking match limit was reached.
PCRE2_ERROR_NOMEMORY
Heap memory is used to remember backtracking points. This error is given when the memory allocation function (default or custom) fails. Note that a different error, PCRE2_ERROR_HEAPLIMIT, is given if the amount of memory needed exceeds the heap limit. PCRE2_ERROR_NOMEMORY is also returned if PCRE2_COPY_MATCHED_SUBJECT is set and memory alloca- tion fails.
PCRE2_ERROR_NULL
Either the code, subject, or match_data argument was passed as NULL.
PCRE2_ERROR_RECURSELOOP
This error is returned when pcre2_match() detects a recursion loop within the pattern. Specifically, it means that either the whole pat- tern or a capture group has been called recursively for the second time at the same position in the subject string. Some simple patterns that might do this are detected and faulted at compile time, but more com- plicated cases, in particular mutual recursions between two different groups, cannot be detected until matching is attempted.
OBTAINING A TEXTUAL ERROR MESSAGE
int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer, PCRE2_SIZE bufflen);
A text message for an error code from any PCRE2 function (compile, match, or auxiliary) can be obtained by calling pcre2_get_error_mes- sage(). The code is passed as the first argument, with the remaining two arguments specifying a code unit buffer and its length in code units, into which the text message is placed. The message is returned in code units of the appropriate width for the library that is being used.
The returned message is terminated with a trailing zero, and the func- tion returns the number of code units used, excluding the trailing zero. If the error number is unknown, the negative error code PCRE2_ER- ROR_BADDATA is returned. If the buffer is too small, the message is truncated (but still with a trailing zero), and the negative error code PCRE2_ERROR_NOMEMORY is returned. None of the messages are very long; a buffer size of 120 code units is ample.
EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
int pcre2_substring_length_bynumber(pcre2_match_data *match_data, uint32_t number, PCRE2_SIZE *length);
int pcre2_substring_copy_bynumber(pcre2_match_data *match_data, uint32_t number, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen);
int pcre2_substring_get_bynumber(pcre2_match_data *match_data, uint32_t number, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen);
void pcre2_substring_free(PCRE2_UCHAR *buffer);
Captured substrings can be accessed directly by using the ovector as described above. For convenience, auxiliary functions are provided for extracting captured substrings as new, separate, zero-terminated strings. A substring that contains a binary zero is correctly extracted and has a further zero added on the end, but the result is not, of course, a C string.
The functions in this section identify substrings by number. The number zero refers to the entire matched substring, with higher numbers refer- ring to substrings captured by parenthesized groups. After a partial match, only substring zero is available. An attempt to extract any other substring gives the error PCRE2_ERROR_PARTIAL. The next section describes similar functions for extracting captured substrings by name.
If a pattern uses the \K escape sequence within a positive assertion, the reported start of a successful match can be greater than the end of the match. For example, if the pattern (?=ab\K) is matched against "ab", the start and end offset values for the match are 2 and 0. In this situation, calling these functions with a zero substring number extracts a zero-length empty string.
You can find the length in code units of a captured substring without extracting it by calling pcre2_substring_length_bynumber(). The first argument is a pointer to the match data block, the second is the group number, and the third is a pointer to a variable into which the length is placed. If you just want to know whether or not the substring has been captured, you can pass the third argument as NULL.
The pcre2_substring_copy_bynumber() function copies a captured sub- string into a supplied buffer, whereas pcre2_substring_get_bynumber() copies it into new memory, obtained using the same memory allocation function that was used for the match data block. The first two argu- ments of these functions are a pointer to the match data block and a capture group number.
The final arguments of pcre2_substring_copy_bynumber() are a pointer to the buffer and a pointer to a variable that contains its length in code units. This is updated to contain the actual number of code units used for the extracted substring, excluding the terminating zero.
For pcre2_substring_get_bynumber() the third and fourth arguments point to variables that are updated with a pointer to the new memory and the number of code units that comprise the substring, again excluding the terminating zero. When the substring is no longer needed, the memory should be freed by calling pcre2_substring_free().
The return value from all these functions is zero for success, or a negative error code. If the pattern match failed, the match failure code is returned. If a substring number greater than zero is used af- ter a partial match, PCRE2_ERROR_PARTIAL is returned. Other possible error codes are:
PCRE2_ERROR_NOMEMORY
The buffer was too small for pcre2_substring_copy_bynumber(), or the attempt to get memory failed for pcre2_substring_get_bynumber().
PCRE2_ERROR_NOSUBSTRING
There is no substring with that number in the pattern, that is, the number is greater than the number of capturing parentheses.
PCRE2_ERROR_UNAVAILABLE
The substring number, though not greater than the number of captures in the pattern, is greater than the number of slots in the ovector, so the substring could not be captured.
PCRE2_ERROR_UNSET
The substring did not participate in the match. For example, if the pattern is (abc)|(def) and the subject is "def", and the ovector con- tains at least two capturing slots, substring number 1 is unset.
EXTRACTING A LIST OF ALL CAPTURED SUBSTRINGS
int pcre2_substring_list_get(pcre2_match_data *match_data, PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr);
void pcre2_substring_list_free(PCRE2_UCHAR **list);
The pcre2_substring_list_get() function extracts all available sub- strings and builds a list of pointers to them. It also (optionally) builds a second list that contains their lengths (in code units), ex- cluding a terminating zero that is added to each of them. All this is done in a single block of memory that is obtained using the same memory allocation function that was used to get the match data block.
This function must be called only after a successful match. If called after a partial match, the error code PCRE2_ERROR_PARTIAL is returned.
The address of the memory block is returned via listptr, which is also the start of the list of string pointers. The end of the list is marked by a NULL pointer. The address of the list of lengths is returned via lengthsptr. If your strings do not contain binary zeros and you do not therefore need the lengths, you may supply NULL as the lengthsptr argu- ment to disable the creation of a list of lengths. The yield of the function is zero if all went well, or PCRE2_ERROR_NOMEMORY if the mem- ory block could not be obtained. When the list is no longer needed, it should be freed by calling pcre2_substring_list_free().
If this function encounters a substring that is unset, which can happen when capture group number n+1 matches some part of the subject, but group n has not been used at all, it returns an empty string. This can be distinguished from a genuine zero-length substring by inspecting the appropriate offset in the ovector, which contain PCRE2_UNSET for unset substrings, or by calling pcre2_substring_length_bynumber().
EXTRACTING CAPTURED SUBSTRINGS BY NAME
int pcre2_substring_number_from_name(const pcre2_code *code, PCRE2_SPTR name);
int pcre2_substring_length_byname(pcre2_match_data *match_data, PCRE2_SPTR name, PCRE2_SIZE *length);
int pcre2_substring_copy_byname(pcre2_match_data *match_data, PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen);
int pcre2_substring_get_byname(pcre2_match_data *match_data, PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen);
void pcre2_substring_free(PCRE2_UCHAR *buffer);
To extract a substring by name, you first have to find associated num- ber. For example, for this pattern:
(a+)b(?<xxx>\d+)...
the number of the capture group called "xxx" is 2. If the name is known to be unique (PCRE2_DUPNAMES was not set), you can find the number from the name by calling pcre2_substring_number_from_name(). The first argu- ment is the compiled pattern, and the second is the name. The yield of the function is the group number, PCRE2_ERROR_NOSUBSTRING if there is no group with that name, or PCRE2_ERROR_NOUNIQUESUBSTRING if there is more than one group with that name. Given the number, you can extract the substring directly from the ovector, or use one of the "bynumber" functions described above.
For convenience, there are also "byname" functions that correspond to the "bynumber" functions, the only difference being that the second ar- gument is a name instead of a number. If PCRE2_DUPNAMES is set and there are duplicate names, these functions scan all the groups with the given name, and return the captured substring from the first named group that is set.
If there are no groups with the given name, PCRE2_ERROR_NOSUBSTRING is returned. If all groups with the name have numbers that are greater than the number of slots in the ovector, PCRE2_ERROR_UNAVAILABLE is re- turned. If there is at least one group with a slot in the ovector, but no group is found to be set, PCRE2_ERROR_UNSET is returned.
Warning: If the pattern uses the (?| feature to set up multiple capture groups with the same number, as described in the section on duplicate group numbers in the pcre2pattern page, you cannot use names to distin- guish the different capture groups, because names are not included in the compiled code. The matching process uses only numbers. For this reason, the use of different names for groups with the same number causes an error at compile time.
CREATING A NEW STRING WITH SUBSTITUTIONS
int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext, PCRE2_SPTR replacement, PCRE2_SIZE rlength, PCRE2_UCHAR *outputbuffer, PCRE2_SIZE *outlengthptr);
This function optionally calls pcre2_match() and then makes a copy of the subject string in outputbuffer, replacing parts that were matched with the replacement string, whose length is supplied in rlength, which can be given as PCRE2_ZERO_TERMINATED for a zero-terminated string. As a special case, if replacement is NULL and rlength is zero, the re- placement is assumed to be an empty string. If rlength is non-zero, an error occurs if replacement is NULL.
There is an option (see PCRE2_SUBSTITUTE_REPLACEMENT_ONLY below) to re- turn just the replacement string(s). The default action is to perform just one replacement if the pattern matches, but there is an option that requests multiple replacements (see PCRE2_SUBSTITUTE_GLOBAL be- low).
If successful, pcre2_substitute() returns the number of substitutions that were carried out. This may be zero if no match was found, and is never greater than one unless PCRE2_SUBSTITUTE_GLOBAL is set. A nega- tive value is returned if an error is detected.
Matches in which a \K item in a lookahead in the pattern causes the match to end before it starts are not supported, and give rise to an error return. For global replacements, matches in which \K in a lookbe- hind causes the match to start earlier than the point that was reached in the previous iteration are also not supported.
The first seven arguments of pcre2_substitute() are the same as for pcre2_match(), except that the partial matching options are not permit- ted, and match_data may be passed as NULL, in which case a match data block is obtained and freed within this function, using memory manage- ment functions from the match context, if provided, or else those that were used to allocate memory for the compiled code.
If match_data is not NULL and PCRE2_SUBSTITUTE_MATCHED is not set, the provided block is used for all calls to pcre2_match(), and its contents afterwards are the result of the final call. For global changes, this will always be a no-match error. The contents of the ovector within the match data block may or may not have been changed.
As well as the usual options for pcre2_match(), a number of additional options can be set in the options argument of pcre2_substitute(). One such option is PCRE2_SUBSTITUTE_MATCHED. When this is set, an external match_data block must be provided, and it must have already been used for an external call to pcre2_match() with the same pattern and subject arguments. The data in the match_data block (return code, offset vec- tor) is then used for the first substitution instead of calling pcre2_match() from within pcre2_substitute(). This allows an applica- tion to check for a match before choosing to substitute, without having to repeat the match.
The contents of the externally supplied match data block are not changed when PCRE2_SUBSTITUTE_MATCHED is set. If PCRE2_SUBSTI- TUTE_GLOBAL is also set, pcre2_match() is called after the first sub- stitution to check for further matches, but this is done using an in- ternally obtained match data block, thus always leaving the external block unchanged.
The code argument is not used for matching before the first substitu- tion when PCRE2_SUBSTITUTE_MATCHED is set, but it must be provided, even when PCRE2_SUBSTITUTE_GLOBAL is not set, because it contains in- formation such as the UTF setting and the number of capturing parenthe- ses in the pattern.
The default action of pcre2_substitute() is to return a copy of the subject string with matched substrings replaced. However, if PCRE2_SUB- STITUTE_REPLACEMENT_ONLY is set, only the replacement substrings are returned. In the global case, multiple replacements are concatenated in the output buffer. Substitution callouts (see below) can be used to separate them if necessary.
The outlengthptr argument of pcre2_substitute() must point to a vari- able that contains the length, in code units, of the output buffer. If the function is successful, the value is updated to contain the length in code units of the new string, excluding the trailing zero that is automatically added.
If the function is not successful, the value set via outlengthptr de- pends on the type of error. For syntax errors in the replacement string, the value is the offset in the replacement string where the er- ror was detected. For other errors, the value is PCRE2_UNSET by de- fault. This includes the case of the output buffer being too small, un- less PCRE2_SUBSTITUTE_OVERFLOW_LENGTH is set.
PCRE2_SUBSTITUTE_OVERFLOW_LENGTH changes what happens when the output buffer is too small. The default action is to return PCRE2_ERROR_NOMEM- ORY immediately. If this option is set, however, pcre2_substitute() continues to go through the motions of matching and substituting (with- out, of course, writing anything) in order to compute the size of buffer that is needed. This value is passed back via the outlengthptr variable, with the result of the function still being PCRE2_ER- ROR_NOMEMORY.
Passing a buffer size of zero is a permitted way of finding out how much memory is needed for given substitution. However, this does mean that the entire operation is carried out twice. Depending on the appli- cation, it may be more efficient to allocate a large buffer and free the excess afterwards, instead of using PCRE2_SUBSTITUTE_OVER- FLOW_LENGTH.
The replacement string, which is interpreted as a UTF string in UTF mode, is checked for UTF validity unless PCRE2_NO_UTF_CHECK is set. An invalid UTF replacement string causes an immediate return with the rel- evant UTF error code.
If PCRE2_SUBSTITUTE_LITERAL is set, the replacement string is not in- terpreted in any way. By default, however, a dollar character is an es- cape character that can specify the insertion of characters from cap- ture groups and names from (*MARK) or other control verbs in the pat- tern. Dollar is the only escape character (backslash is treated as lit- eral). The following forms are always recognized:
$$ insert a dollar character $<n> or ${<n>} insert the contents of group <n> $*MARK or ${*MARK} insert a control verb name
Either a group number or a group name can be given for <n>. Curly brackets are required only if the following character would be inter- preted as part of the number or name. The number may be zero to include the entire matched string. For example, if the pattern a(b)c is matched with "=abc=" and the replacement string "+$1$0$1+", the result is "=+babcb+=".
$*MARK inserts the name from the last encountered backtracking control verb on the matching path that has a name. (*MARK) must always include a name, but the other verbs need not. For example, in the case of (*MARK:A)(*PRUNE) the name inserted is "A", but for (*MARK:A)(*PRUNE:B) the relevant name is "B". This facility can be used to perform simple simultaneous substitutions, as this pcre2test example shows:
/(*MARK:pear)apple|(*MARK:orange)lemon/g,replace=${*MARK} apple lemon 2: pear orange
PCRE2_SUBSTITUTE_GLOBAL causes the function to iterate over the subject string, replacing every matching substring. If this option is not set, only the first matching substring is replaced. The search for matches takes place in the original subject string (that is, previous replace- ments do not affect it). Iteration is implemented by advancing the startoffset value for each search, which is always passed the entire subject string. If an offset limit is set in the match context, search- ing stops when that limit is reached.
You can restrict the effect of a global substitution to a portion of the subject string by setting either or both of startoffset and an off- set limit. Here is a pcre2test example:
/B/g,replace=!,use_offset_limit ABC ABC ABC ABC\=offset=3,offset_limit=12 2: ABC A!C A!C ABC
When continuing with global substitutions after matching a substring with zero length, an attempt to find a non-empty match at the same off- set is performed. If this is not successful, the offset is advanced by one character except when CRLF is a valid newline sequence and the next two characters are CR, LF. In this case, the offset is advanced by two characters.
PCRE2_SUBSTITUTE_UNKNOWN_UNSET causes references to capture groups that do not appear in the pattern to be treated as unset groups. This option should be used with care, because it means that a typo in a group name or number no longer causes the PCRE2_ERROR_NOSUBSTRING error.
PCRE2_SUBSTITUTE_UNSET_EMPTY causes unset capture groups (including un- known groups when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set) to be treated as empty strings when inserted as described above. If this option is not set, an attempt to insert an unset group causes the PCRE2_ERROR_UN- SET error. This option does not influence the extended substitution syntax described below.
PCRE2_SUBSTITUTE_EXTENDED causes extra processing to be applied to the replacement string. Without this option, only the dollar character is special, and only the group insertion forms listed above are valid. When PCRE2_SUBSTITUTE_EXTENDED is set, two things change:
Firstly, backslash in a replacement string is interpreted as an escape character. The usual forms such as \n or \x{ddd} can be used to specify particular character codes, and backslash followed by any non-alphanu- meric character quotes that character. Extended quoting can be coded using \Q...\E, exactly as in pattern strings.
There are also four escape sequences for forcing the case of inserted letters. The insertion mechanism has three states: no case forcing, force upper case, and force lower case. The escape sequences change the current state: \U and \L change to upper or lower case forcing, respec- tively, and \E (when not terminating a \Q quoted sequence) reverts to no case forcing. The sequences \u and \l force the next character (if it is a letter) to upper or lower case, respectively, and then the state automatically reverts to no case forcing. Case forcing applies to all inserted characters, including those from capture groups and let- ters within \Q...\E quoted sequences. If either PCRE2_UTF or PCRE2_UCP was set when the pattern was compiled, Unicode properties are used for case forcing characters whose code points are greater than 127.
Note that case forcing sequences such as \U...\E do not nest. For exam- ple, the result of processing "\Uaa\LBB\Ecc\E" is "AAbbcc"; the final \E has no effect. Note also that the PCRE2_ALT_BSUX and PCRE2_EX- TRA_ALT_BSUX options do not apply to replacement strings.
The second effect of setting PCRE2_SUBSTITUTE_EXTENDED is to add more flexibility to capture group substitution. The syntax is similar to that used by Bash:
${<n>:-<string>} ${<n>:+<string1>:<string2>}
As before, <n> may be a group number or a name. The first form speci- fies a default value. If group <n> is set, its value is inserted; if not, <string> is expanded and the result inserted. The second form specifies strings that are expanded and inserted when group <n> is set or unset, respectively. The first form is just a convenient shorthand for
${<n>:+${<n>}:<string>}
Backslash can be used to escape colons and closing curly brackets in the replacement strings. A change of the case forcing state within a replacement string remains in force afterwards, as shown in this pcre2test example:
/(some)?(body)/substitute_extended,replace=${1:+\U:\L}HeLLo body 1: hello somebody 1: HELLO
The PCRE2_SUBSTITUTE_UNSET_EMPTY option does not affect these extended substitutions. However, PCRE2_SUBSTITUTE_UNKNOWN_UNSET does cause un- known groups in the extended syntax forms to be treated as unset.
If PCRE2_SUBSTITUTE_LITERAL is set, PCRE2_SUBSTITUTE_UNKNOWN_UNSET, PCRE2_SUBSTITUTE_UNSET_EMPTY, and PCRE2_SUBSTITUTE_EXTENDED are irrele- vant and are ignored.
Substitution errors
In the event of an error, pcre2_substitute() returns a negative error code. Except for PCRE2_ERROR_NOMATCH (which is never returned), errors from pcre2_match() are passed straight back.
PCRE2_ERROR_NOSUBSTRING is returned for a non-existent substring inser- tion, unless PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set.
PCRE2_ERROR_UNSET is returned for an unset substring insertion (includ- ing an unknown substring when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set) when the simple (non-extended) syntax is used and PCRE2_SUBSTITUTE_UN- SET_EMPTY is not set.
PCRE2_ERROR_NOMEMORY is returned if the output buffer is not big enough. If the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option is set, the size of buffer that is needed is returned via outlengthptr. Note that this does not happen by default.
PCRE2_ERROR_NULL is returned if PCRE2_SUBSTITUTE_MATCHED is set but the match_data argument is NULL or if the subject or replacement arguments are NULL. For backward compatibility reasons an exception is made for the replacement argument if the rlength argument is also 0.
PCRE2_ERROR_BADREPLACEMENT is used for miscellaneous syntax errors in the replacement string, with more particular errors being PCRE2_ER- ROR_BADREPESCAPE (invalid escape sequence), PCRE2_ERROR_REPMISSINGBRACE (closing curly bracket not found), PCRE2_ERROR_BADSUBSTITUTION (syntax error in extended group substitution), and PCRE2_ERROR_BADSUBSPATTERN (the pattern match ended before it started or the match started earlier than the current position in the subject, which can happen if \K is used in an assertion).
As for all PCRE2 errors, a text message that describes the error can be obtained by calling the pcre2_get_error_message() function (see "Ob- taining a textual error message" above).
Substitution callouts
int pcre2_set_substitute_callout(pcre2_match_context *mcontext, int (*callout_function)(pcre2_substitute_callout_block *, void *), void *callout_data);
The pcre2_set_substitution_callout() function can be used to specify a callout function for pcre2_substitute(). This information is passed in a match context. The callout function is called after each substitution has been processed, but it can cause the replacement not to happen. The callout function is not called for simulated substitutions that happen as a result of the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option.
The first argument of the callout function is a pointer to a substitute callout block structure, which contains the following fields, not nec- essarily in this order:
uint32_t version; uint32_t subscount; PCRE2_SPTR input; PCRE2_SPTR output; PCRE2_SIZE *ovector; uint32_t oveccount; PCRE2_SIZE output_offsets[2];
The version field contains the version number of the block format. The current version is 0. The version number will increase in future if more fields are added, but the intention is never to remove any of the existing fields.
The subscount field is the number of the current match. It is 1 for the first callout, 2 for the second, and so on. The input and output point- ers are copies of the values passed to pcre2_substitute().
The ovector field points to the ovector, which contains the result of the most recent match. The oveccount field contains the number of pairs that are set in the ovector, and is always greater than zero.
The output_offsets vector contains the offsets of the replacement in the output string. This has already been processed for dollar and (if requested) backslash substitutions as described above.
The second argument of the callout function is the value passed as callout_data when the function was registered. The value returned by the callout function is interpreted as follows:
If the value is zero, the replacement is accepted, and, if PCRE2_SUB- STITUTE_GLOBAL is set, processing continues with a search for the next match. If the value is not zero, the current replacement is not ac- cepted. If the value is greater than zero, processing continues when PCRE2_SUBSTITUTE_GLOBAL is set. Otherwise (the value is less than zero or PCRE2_SUBSTITUTE_GLOBAL is not set), the rest of the input is copied to the output and the call to pcre2_substitute() exits, returning the number of matches so far.
DUPLICATE CAPTURE GROUP NAMES
int pcre2_substring_nametable_scan(const pcre2_code *code, PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last);
When a pattern is compiled with the PCRE2_DUPNAMES option, names for capture groups are not required to be unique. Duplicate names are al- ways allowed for groups with the same number, created by using the (?| feature. Indeed, if such groups are named, they are required to use the same names.
Normally, patterns that use duplicate names are such that in any one match, only one of each set of identically-named groups participates. An example is shown in the pcre2pattern documentation.
When duplicates are present, pcre2_substring_copy_byname() and pcre2_substring_get_byname() return the first substring corresponding to the given name that is set. Only if none are set is PCRE2_ERROR_UN- SET is returned. The pcre2_substring_number_from_name() function re- turns the error PCRE2_ERROR_NOUNIQUESUBSTRING when there are duplicate names.
If you want to get full details of all captured substrings for a given name, you must use the pcre2_substring_nametable_scan() function. The first argument is the compiled pattern, and the second is the name. If the third and fourth arguments are NULL, the function returns a group number for a unique name, or PCRE2_ERROR_NOUNIQUESUBSTRING otherwise.
When the third and fourth arguments are not NULL, they must be pointers to variables that are updated by the function. After it has run, they point to the first and last entries in the name-to-number table for the given name, and the function returns the length of each entry in code units. In both cases, PCRE2_ERROR_NOSUBSTRING is returned if there are no entries for the given name.
The format of the name table is described above in the section entitled Information about a pattern. Given all the relevant entries for the name, you can extract each of their numbers, and hence the captured data.
FINDING ALL POSSIBLE MATCHES AT ONE POSITION
The traditional matching function uses a similar algorithm to Perl, which stops when it finds the first match at a given point in the sub- ject. If you want to find all possible matches, or the longest possible match at a given position, consider using the alternative matching function (see below) instead. If you cannot use the alternative func- tion, you can kludge it up by making use of the callout facility, which is described in the pcre2callout documentation.
What you have to do is to insert a callout right at the end of the pat- tern. When your callout function is called, extract and save the cur- rent matched substring. Then return 1, which forces pcre2_match() to backtrack and try other alternatives. Ultimately, when it runs out of matches, pcre2_match() will yield PCRE2_ERROR_NOMATCH.
MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject, PCRE2_SIZE length, PCRE2_SIZE startoffset, uint32_t options, pcre2_match_data *match_data, pcre2_match_context *mcontext, int *workspace, PCRE2_SIZE wscount);
The function pcre2_dfa_match() is called to match a subject string against a compiled pattern, using a matching algorithm that scans the subject string just once (not counting lookaround assertions), and does not backtrack (except when processing lookaround assertions). This has different characteristics to the normal algorithm, and is not compati- ble with Perl. Some of the features of PCRE2 patterns are not sup- ported. Nevertheless, there are times when this kind of matching can be useful. For a discussion of the two matching algorithms, and a list of features that pcre2_dfa_match() does not support, see the pcre2matching documentation.
The arguments for the pcre2_dfa_match() function are the same as for pcre2_match(), plus two extras. The ovector within the match data block is used in a different way, and this is described below. The other com- mon arguments are used in the same way as for pcre2_match(), so their description is not repeated here.
The two additional arguments provide workspace for the function. The workspace vector should contain at least 20 elements. It is used for keeping track of multiple paths through the pattern tree. More work- space is needed for patterns and subjects where there are a lot of po- tential matches.
Here is an example of a simple call to pcre2_dfa_match():
int wspace[20]; pcre2_match_data *md = pcre2_match_data_create(4, NULL); int rc = pcre2_dfa_match( re, /* result of pcre2_compile() */ "some string", /* the subject string */ 11, /* the length of the subject string */ 0, /* start at offset 0 in the subject */ 0, /* default options */ md, /* the match data block */ NULL, /* a match context; NULL means use defaults */ wspace, /* working space vector */ 20); /* number of elements (NOT size in bytes) */
Option bits for pcre2_dfa_match()
The unused bits of the options argument for pcre2_dfa_match() must be zero. The only bits that may be set are PCRE2_ANCHORED, PCRE2_COPY_MATCHED_SUBJECT, PCRE2_ENDANCHORED, PCRE2_NOTBOL, PCRE2_NO- TEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, PCRE2_PARTIAL_SOFT, PCRE2_DFA_SHORTEST, and PCRE2_DFA_RESTART. All but the last four of these are exactly the same as for pcre2_match(), so their description is not repeated here.
PCRE2_PARTIAL_HARD PCRE2_PARTIAL_SOFT
These have the same general effect as they do for pcre2_match(), but the details are slightly different. When PCRE2_PARTIAL_HARD is set for pcre2_dfa_match(), it returns PCRE2_ERROR_PARTIAL if the end of the subject is reached and there is still at least one matching possibility that requires additional characters. This happens even if some complete matches have already been found. When PCRE2_PARTIAL_SOFT is set, the return code PCRE2_ERROR_NOMATCH is converted into PCRE2_ERROR_PARTIAL if the end of the subject is reached, there have been no complete matches, but there is still at least one matching possibility. The por- tion of the string that was inspected when the longest partial match was found is set as the first matching string in both cases. There is a more detailed discussion of partial and multi-segment matching, with examples, in the pcre2partial documentation.
PCRE2_DFA_SHORTEST
Setting the PCRE2_DFA_SHORTEST option causes the matching algorithm to stop as soon as it has found one match. Because of the way the alterna- tive algorithm works, this is necessarily the shortest possible match at the first possible matching point in the subject string.
PCRE2_DFA_RESTART
When pcre2_dfa_match() returns a partial match, it is possible to call it again, with additional subject characters, and have it continue with the same match. The PCRE2_DFA_RESTART option requests this action; when it is set, the workspace and wscount options must reference the same vector as before because data about the match so far is left in them after a partial match. There is more discussion of this facility in the pcre2partial documentation.
Successful returns from pcre2_dfa_match()
When pcre2_dfa_match() succeeds, it may have matched more than one sub- string in the subject. Note, however, that all the matches from one run of the function start at the same point in the subject. The shorter matches are all initial substrings of the longer matches. For example, if the pattern
<.*>
is matched against the string
This is <something> <something else> <something further> no more
the three matched strings are
<something> <something else> <something further> <something> <something else> <something>
On success, the yield of the function is a number greater than zero, which is the number of matched substrings. The offsets of the sub- strings are returned in the ovector, and can be extracted by number in the same way as for pcre2_match(), but the numbers bear no relation to any capture groups that may exist in the pattern, because DFA matching does not support capturing.
Calls to the convenience functions that extract substrings by name re- turn the error PCRE2_ERROR_DFA_UFUNC (unsupported function) if used af- ter a DFA match. The convenience functions that extract substrings by number never return PCRE2_ERROR_NOSUBSTRING.
The matched strings are stored in the ovector in reverse order of length; that is, the longest matching string is first. If there were too many matches to fit into the ovector, the yield of the function is zero, and the vector is filled with the longest matches.
NOTE: PCRE2's "auto-possessification" optimization usually applies to character repeats at the end of a pattern (as well as internally). For example, the pattern "a\d+" is compiled as if it were "a\d++". For DFA matching, this means that only one possible match is found. If you re- ally do want multiple matches in such cases, either use an ungreedy re- peat such as "a\d+?" or set the PCRE2_NO_AUTO_POSSESS option when com- piling.
Error returns from pcre2_dfa_match()
The pcre2_dfa_match() function returns a negative number when it fails. Many of the errors are the same as for pcre2_match(), as described above. There are in addition the following errors that are specific to pcre2_dfa_match():
PCRE2_ERROR_DFA_UITEM
This return is given if pcre2_dfa_match() encounters an item in the pattern that it does not support, for instance, the use of \C in a UTF mode or a backreference.
PCRE2_ERROR_DFA_UCOND
This return is given if pcre2_dfa_match() encounters a condition item that uses a backreference for the condition, or a test for recursion in a specific capture group. These are not supported.
PCRE2_ERROR_DFA_UINVALID_UTF
This return is given if pcre2_dfa_match() is called for a pattern that was compiled with PCRE2_MATCH_INVALID_UTF. This is not supported for DFA matching.
PCRE2_ERROR_DFA_WSSIZE
This return is given if pcre2_dfa_match() runs out of space in the workspace vector.
PCRE2_ERROR_DFA_RECURSE
When a recursion or subroutine call is processed, the matching function calls itself recursively, using private memory for the ovector and workspace. This error is given if the internal ovector is not large enough. This should be extremely rare, as a vector of size 1000 is used.
PCRE2_ERROR_DFA_BADRESTART
When pcre2_dfa_match() is called with the PCRE2_DFA_RESTART option, some plausibility checks are made on the contents of the workspace, which should contain data about the previous partial match. If any of these checks fail, this error is given.
SEE ALSO
pcre2build(3), pcre2callout(3), pcre2demo(3), pcre2matching(3), pcre2partial(3), pcre2posix(3), pcre2sample(3), pcre2unicode(3).
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
REVISION
Last updated: 24 April 2024 Copyright (c) 1997-2024 University of Cambridge.
PCRE2 10.44 24 April 2024 PCRE2API(3)------------------------------------------------------------------------------
PCRE2BUILD(3) Library Functions Manual PCRE2BUILD(3)
NAME PCRE2 - Perl-compatible regular expressions (revised API)
BUILDING PCRE2
PCRE2 is distributed with a configure script that can be used to build the library in Unix-like environments using the applications known as Autotools. Also in the distribution are files to support building using CMake instead of configure. The text file README contains general in- formation about building with Autotools (some of which is repeated be- low), and also has some comments about building on various operating systems. The files in the vms directory support building under OpenVMS. There is a lot more information about building PCRE2 without using Au- totools (including information about using CMake and building "by hand") in the text file called NON-AUTOTOOLS-BUILD. You should consult this file as well as the README file if you are building in a non-Unix- like environment.
PCRE2 BUILD-TIME OPTIONS
The rest of this document describes the optional features of PCRE2 that can be selected when the library is compiled. It assumes use of the configure script, where the optional features are selected or dese- lected by providing options to configure before running the make com- mand. However, the same options can be selected in both Unix-like and non-Unix-like environments if you are using CMake instead of configure to build PCRE2.
If you are not using Autotools or CMake, option selection can be done by editing the config.h file, or by passing parameter settings to the compiler, as described in NON-AUTOTOOLS-BUILD.
The complete list of options for configure (which includes the standard ones such as the selection of the installation directory) can be ob- tained by running
./configure --help
The following sections include descriptions of "on/off" options whose names begin with --enable or --disable. Because of the way that config- ure works, --enable and --disable always come in pairs, so the comple- mentary option always exists as well, but as it specifies the default, it is not described. Options that specify values have names that start with --with. At the end of a configure run, a summary of the configura- tion is output.
BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES
By default, a library called libpcre2-8 is built, containing functions that take string arguments contained in arrays of bytes, interpreted either as single-byte characters, or UTF-8 strings. You can also build two other libraries, called libpcre2-16 and libpcre2-32, which process strings that are contained in arrays of 16-bit and 32-bit code units, respectively. These can be interpreted either as single-unit characters or UTF-16/UTF-32 strings. To build these additional libraries, add one or both of the following to the configure command:
--enable-pcre2-16 --enable-pcre2-32
If you do not want the 8-bit library, add
--disable-pcre2-8
as well. At least one of the three libraries must be built. Note that the POSIX wrapper is for the 8-bit library only, and that pcre2grep is an 8-bit program. Neither of these are built if you select only the 16-bit or 32-bit libraries.
BUILDING SHARED AND STATIC LIBRARIES
The Autotools PCRE2 building process uses libtool to build both shared and static libraries by default. You can suppress an unwanted library by adding one of
--disable-shared --disable-static
to the configure command. Setting --disable-shared ensures that PCRE2 libraries are built as static libraries. The binaries that are then created as part of the build process (for example, pcre2test and pcre2grep) are linked statically with one or more PCRE2 libraries, but may also be dynamically linked with other libraries such as libc. If you want these binaries to be fully statically linked, you can set LD- FLAGS like this:
LDFLAGS=--static ./configure --disable-shared
Note the two hyphens in --static. Of course, this works only if static versions of all the relevant libraries are available for linking.
UNICODE AND UTF SUPPORT
By default, PCRE2 is built with support for Unicode and UTF character strings. To build it without Unicode support, add
--disable-unicode
to the configure command. This setting applies to all three libraries. It is not possible to build one library with Unicode support and an- other without in the same configuration.
Of itself, Unicode support does not make PCRE2 treat strings as UTF-8, UTF-16 or UTF-32. To do that, applications that use the library can set the PCRE2_UTF option when they call pcre2_compile() to compile a pat- tern. Alternatively, patterns may be started with (*UTF) unless the application has locked this out by setting PCRE2_NEVER_UTF.
UTF support allows the libraries to process character code points up to 0x10ffff in the strings that they handle. Unicode support also gives access to the Unicode properties of characters, using pattern escapes such as \P, \p, and \X. Only the general category properties such as Lu and Nd, script names, and some bi-directional properties are supported. Details are given in the pcre2pattern documentation.
Pattern escapes such as \d and \w do not by default make use of Unicode properties. The application can request that they do by setting the PCRE2_UCP option. Unless the application has set PCRE2_NEVER_UCP, a pattern may also request this by starting with (*UCP).
DISABLING THE USE OF \C
The \C escape sequence, which matches a single code unit, even in a UTF mode, can cause unpredictable behaviour because it may leave the cur- rent matching point in the middle of a multi-code-unit character. The application can lock it out by setting the PCRE2_NEVER_BACKSLASH_C op- tion when calling pcre2_compile(). There is also a build-time option
--enable-never-backslash-C
(note the upper case C) which locks out the use of \C entirely.
JUST-IN-TIME COMPILER SUPPORT
Just-in-time (JIT) compiler support is included in the build by speci- fying
--enable-jit
This support is available only for certain hardware architectures. If this option is set for an unsupported architecture, a building error occurs. If in doubt, use
--enable-jit=auto
which enables JIT only if the current hardware is supported. You can check if JIT is enabled in the configuration summary that is output at the end of a configure run. If you are enabling JIT under SELinux you may also want to add
--enable-jit-sealloc
which enables the use of an execmem allocator in JIT that is compatible with SELinux. This has no effect if JIT is not enabled. See the pcre2jit documentation for a discussion of JIT usage. When JIT support is enabled, pcre2grep automatically makes use of it, unless you add
--disable-pcre2grep-jit
to the configure command.
NEWLINE RECOGNITION
By default, PCRE2 interprets the linefeed (LF) character as indicating the end of a line. This is the normal newline character on Unix-like systems. You can compile PCRE2 to use carriage return (CR) instead, by adding
--enable-newline-is-cr
to the configure command. There is also an --enable-newline-is-lf op- tion, which explicitly specifies linefeed as the newline character.
Alternatively, you can specify that line endings are to be indicated by the two-character sequence CRLF (CR immediately followed by LF). If you want this, add
--enable-newline-is-crlf
to the configure command. There is a fourth option, specified by
--enable-newline-is-anycrlf
which causes PCRE2 to recognize any of the three sequences CR, LF, or CRLF as indicating a line ending. A fifth option, specified by
--enable-newline-is-any
causes PCRE2 to recognize any Unicode newline sequence. The Unicode newline sequences are the three just mentioned, plus the single charac- ters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS (paragraph separator, U+2029). The final option is
--enable-newline-is-nul
which causes NUL (binary zero) to be set as the default line-ending character.
Whatever default line ending convention is selected when PCRE2 is built can be overridden by applications that use the library. At build time it is recommended to use the standard for your operating system.
WHAT \R MATCHES
By default, the sequence \R in a pattern matches any Unicode newline sequence, independently of what has been selected as the line ending sequence. If you specify
--enable-bsr-anycrlf
the default is changed so that \R matches only CR, LF, or CRLF. What- ever is selected when PCRE2 is built can be overridden by applications that use the library.
HANDLING VERY LARGE PATTERNS
Within a compiled pattern, offset values are used to point from one part to another (for example, from an opening parenthesis to an alter- nation metacharacter). By default, in the 8-bit and 16-bit libraries, two-byte values are used for these offsets, leading to a maximum size for a compiled pattern of around 64 thousand code units. This is suffi- cient to handle all but the most gigantic patterns. Nevertheless, some people do want to process truly enormous patterns, so it is possible to compile PCRE2 to use three-byte or four-byte offsets by adding a set- ting such as
--with-link-size=3
to the configure command. The value given must be 2, 3, or 4. For the 16-bit library, a value of 3 is rounded up to 4. In these libraries, using longer offsets slows down the operation of PCRE2 because it has to load additional data when handling them. For the 32-bit library the value is always 4 and cannot be overridden; the value of --with-link- size is ignored.
LIMITING PCRE2 RESOURCE USAGE
The pcre2_match() function increments a counter each time it goes round its main loop. Putting a limit on this counter controls the amount of computing resource used by a single call to pcre2_match(). The limit can be changed at run time, as described in the pcre2api documentation. The default is 10 million, but this can be changed by adding a setting such as
--with-match-limit=500000
to the configure command. This setting also applies to the pcre2_dfa_match() matching function, and to JIT matching (though the counting is done differently).
The pcre2_match() function uses heap memory to record backtracking points. The more nested backtracking points there are (that is, the deeper the search tree), the more memory is needed. There is an upper limit, specified in kibibytes (units of 1024 bytes). This limit can be changed at run time, as described in the pcre2api documentation. The default limit (in effect unlimited) is 20 million. You can change this by a setting such as
--with-heap-limit=500
which limits the amount of heap to 500 KiB. This limit applies only to interpretive matching in pcre2_match() and pcre2_dfa_match(), which may also use the heap for internal workspace when processing complicated patterns. This limit does not apply when JIT (which has its own memory arrangements) is used.
You can also explicitly limit the depth of nested backtracking in the pcre2_match() interpreter. This limit defaults to the value that is set for --with-match-limit. You can set a lower default limit by adding, for example,
--with-match-limit-depth=10000
to the configure command. This value can be overridden at run time. This depth limit indirectly limits the amount of heap memory that is used, but because the size of each backtracking "frame" depends on the number of capturing parentheses in a pattern, the amount of heap that is used before the limit is reached varies from pattern to pattern. This limit was more useful in versions before 10.30, where function re- cursion was used for backtracking.
As well as applying to pcre2_match(), the depth limit also controls the depth of recursive function calls in pcre2_dfa_match(). These are used for lookaround assertions, atomic groups, and recursion within pat- terns. The limit does not apply to JIT matching.
LIMITING VARIABLE-LENGTH LOOKBEHIND ASSERTIONS
Lookbehind assertions in which one or more branches can match a vari- able number of characters are supported only if there is a maximum matching length for each top-level branch. There is a limit to this maximum that defaults to 255 characters. You can alter this default by a setting such as
--with-max-varlookbehind=100
The limit can be changed at runtime by calling pcre2_set_max_varlookbe- hind(). Lookbehind assertions in which every branch matches a fixed number of characters (not necessarily all the same) are not constrained by this limit.
CREATING CHARACTER TABLES AT BUILD TIME
PCRE2 uses fixed tables for processing characters whose code points are less than 256. By default, PCRE2 is built with a set of tables that are distributed in the file src/pcre2_chartables.c.dist. These tables are for ASCII codes only. If you add
--enable-rebuild-chartables
to the configure command, the distributed tables are no longer used. Instead, a program called pcre2_dftables is compiled and run. This out- puts the source for new set of tables, created in the default locale of your C run-time system. This method of replacing the tables does not work if you are cross compiling, because pcre2_dftables needs to be run on the local host and therefore not compiled with the cross compiler.
If you need to create alternative tables when cross compiling, you will have to do so "by hand". There may also be other reasons for creating tables manually. To cause pcre2_dftables to be built on the local host, run a normal compiling command, and then run the program with the output file as its argument, for example:
cc src/pcre2_dftables.c -o pcre2_dftables ./pcre2_dftables src/pcre2_chartables.c
This builds the tables in the default locale of the local host. If you want to specify a locale, you must use the -L option:
LC_ALL=fr_FR ./pcre2_dftables -L src/pcre2_chartables.c
You can also specify -b (with or without -L). This causes the tables to be written in binary instead of as source code. A set of binary tables can be loaded into memory by an application and passed to pcre2_com- pile() in the same way as tables created by calling pcre2_maketables(). The tables are just a string of bytes, independent of hardware charac- teristics such as endianness. This means they can be bundled with an application that runs in different environments, to ensure consistent behaviour.
USING EBCDIC CODE
PCRE2 assumes by default that it will run in an environment where the character code is ASCII or Unicode, which is a superset of ASCII. This is the case for most computer operating systems. PCRE2 can, however, be compiled to run in an 8-bit EBCDIC environment by adding
--enable-ebcdic --disable-unicode
to the configure command. This setting implies --enable-rebuild-charta- bles. You should only use it if you know that you are in an EBCDIC en- vironment (for example, an IBM mainframe operating system).
It is not possible to support both EBCDIC and UTF-8 codes in the same version of the library. Consequently, --enable-unicode and --enable- ebcdic are mutually exclusive.
The EBCDIC character that corresponds to an ASCII LF is assumed to have the value 0x15 by default. However, in some EBCDIC environments, 0x25 is used. In such an environment you should use
--enable-ebcdic-nl25
as well as, or instead of, --enable-ebcdic. The EBCDIC character for CR has the same value as in ASCII, namely, 0x0d. Whichever of 0x15 and 0x25 is not chosen as LF is made to correspond to the Unicode NEL char- acter (which, in Unicode, is 0x85).
The options that select newline behaviour, such as --enable-newline-is- cr, and equivalent run-time options, refer to these character values in an EBCDIC environment.
PCRE2GREP SUPPORT FOR EXTERNAL SCRIPTS
By default pcre2grep supports the use of callouts with string arguments within the patterns it is matching. There are two kinds: one that gen- erates output using local code, and another that calls an external pro- gram or script. If --disable-pcre2grep-callout-fork is added to the configure command, only the first kind of callout is supported; if --disable-pcre2grep-callout is used, all callouts are completely ig- nored. For more details of pcre2grep callouts, see the pcre2grep docu- mentation.
PCRE2GREP OPTIONS FOR COMPRESSED FILE SUPPORT
By default, pcre2grep reads all files as plain text. You can build it so that it recognizes files whose names end in .gz or .bz2, and reads them with libz or libbz2, respectively, by adding one or both of
--enable-pcre2grep-libz --enable-pcre2grep-libbz2
to the configure command. These options naturally require that the rel- evant libraries are installed on your system. Configuration will fail if they are not.
PCRE2GREP BUFFER SIZE
pcre2grep uses an internal buffer to hold a "window" on the file it is scanning, in order to be able to output "before" and "after" lines when it finds a match. The default starting size of the buffer is 20KiB. The buffer itself is three times this size, but because of the way it is used for holding "before" lines, the longest line that is guaranteed to be processable is the notional buffer size. If a longer line is encoun- tered, pcre2grep automatically expands the buffer, up to a specified maximum size, whose default is 1MiB or the starting size, whichever is the larger. You can change the default parameter values by adding, for example,
--with-pcre2grep-bufsize=51200 --with-pcre2grep-max-bufsize=2097152
to the configure command. The caller of pcre2grep can override these values by using --buffer-size and --max-buffer-size on the command line.
PCRE2TEST OPTION FOR LIBREADLINE SUPPORT
If you add one of
--enable-pcre2test-libreadline --enable-pcre2test-libedit
to the configure command, pcre2test is linked with the libreadline or- libedit library, respectively, and when its input is from a terminal, it reads it using the readline() function. This provides line-editing and history facilities. Note that libreadline is GPL-licensed, so if you distribute a binary of pcre2test linked in this way, there may be licensing issues. These can be avoided by linking instead with libedit, which has a BSD licence.
Setting --enable-pcre2test-libreadline causes the -lreadline option to be added to the pcre2test build. In many operating environments with a system-installed readline library this is sufficient. However, in some environments (e.g. if an unmodified distribution version of readline is in use), some extra configuration may be necessary. The INSTALL file for libreadline says this:
"Readline uses the termcap functions, but does not link with the termcap or curses library itself, allowing applications which link with readline the to choose an appropriate library."
If your environment has not been set up so that an appropriate library is automatically included, you may need to add something like
LIBS="-ncurses"
immediately before the configure command.
INCLUDING DEBUGGING CODE
If you add
--enable-debug
to the configure command, additional debugging code is included in the build. This feature is intended for use by the PCRE2 maintainers.
DEBUGGING WITH VALGRIND SUPPORT
If you add
--enable-valgrind
to the configure command, PCRE2 will use valgrind annotations to mark certain memory regions as unaddressable. This allows it to detect in- valid memory accesses, and is mostly useful for debugging PCRE2 itself.
CODE COVERAGE REPORTING
If your C compiler is gcc, you can build a version of PCRE2 that can generate a code coverage report for its test suite. To enable this, you must install lcov version 1.6 or above. Then specify
--enable-coverage
to the configure command and build PCRE2 in the usual way.
Note that using ccache (a caching C compiler) is incompatible with code coverage reporting. If you have configured ccache to run automatically on your system, you must set the environment variable
CCACHE_DISABLE=1
before running make to build PCRE2, so that ccache is not used.
When --enable-coverage is used, the following addition targets are added to the Makefile:
make coverage
This creates a fresh coverage report for the PCRE2 test suite. It is equivalent to running "make coverage-reset", "make coverage-baseline", "make check", and then "make coverage-report".
make coverage-reset
This zeroes the coverage counters, but does nothing else.
make coverage-baseline
This captures baseline coverage information.
make coverage-report
This creates the coverage report.
make coverage-clean-report
This removes the generated coverage report without cleaning the cover- age data itself.
make coverage-clean-data
This removes the captured coverage data without removing the coverage files created at compile time (*.gcno).
make coverage-clean
This cleans all coverage data including the generated coverage report. For more information about code coverage, see the gcov and lcov docu- mentation.
DISABLING THE Z AND T FORMATTING MODIFIERS
The C99 standard defines formatting modifiers z and t for size_t and ptrdiff_t values, respectively. By default, PCRE2 uses these modifiers in environments other than old versions of Microsoft Visual Studio when __STDC_VERSION__ is defined and has a value greater than or equal to 199901L (indicating support for C99). However, there is at least one environment that claims to be C99 but does not support these modifiers. If
--disable-percent-zt
is specified, no use is made of the z or t modifiers. Instead of %td or %zu, a suitable format is used depending in the size of long for the platform.
SUPPORT FOR FUZZERS
There is a special option for use by people who want to run fuzzing tests on PCRE2:
--enable-fuzz-support
At present this applies only to the 8-bit library. If set, it causes an extra library called libpcre2-fuzzsupport.a to be built, but not in- stalled. This contains a single function called LLVMFuzzerTestOneIn- put() whose arguments are a pointer to a string and the length of the string. When called, this function tries to compile the string as a pattern, and if that succeeds, to match it. This is done both with no options and with some random options bits that are generated from the string.
Setting --enable-fuzz-support also causes a binary called pcre2fuz- zcheck to be created. This is normally run under valgrind or used when PCRE2 is compiled with address sanitizing enabled. It calls the fuzzing function and outputs information about what it is doing. The input strings are specified by arguments: if an argument starts with "=" the rest of it is a literal input string. Otherwise, it is assumed to be a file name, and the contents of the file are the test string.
OBSOLETE OPTION
In versions of PCRE2 prior to 10.30, there were two ways of handling backtracking in the pcre2_match() function. The default was to use the system stack, but if
--disable-stack-for-recursion
was set, memory on the heap was used. From release 10.30 onwards this has changed (the stack is no longer used) and this option now does nothing except give a warning.
SEE ALSO
pcre2api(3), pcre2-config(3).
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
REVISION
Last updated: 15 April 2024 Copyright (c) 1997-2024 University of Cambridge.
PCRE2 10.44 15 April 2024 PCRE2BUILD(3)------------------------------------------------------------------------------
PCRE2CALLOUT(3) Library Functions Manual PCRE2CALLOUT(3)
NAME PCRE2 - Perl-compatible regular expressions (revised API)
SYNOPSIS
#include <pcre2.h>
int (*pcre2_callout)(pcre2_callout_block *, void *);
int pcre2_callout_enumerate(const pcre2_code *code, int (*callback)(pcre2_callout_enumerate_block *, void *), void *user_data);
DESCRIPTION
PCRE2 provides a feature called "callout", which is a means of tem- porarily passing control to the caller of PCRE2 in the middle of pat- tern matching. The caller of PCRE2 provides an external function by putting its entry point in a match context (see pcre2_set_callout() in the pcre2api documentation).
When using the pcre2_substitute() function, an additional callout fea- ture is available. This does a callout after each change to the subject string and is described in the pcre2api documentation; the rest of this document is concerned with callouts during pattern matching.
Within a regular expression, (?C<arg>) indicates a point at which the external function is to be called. Different callout points can be identified by putting a number less than 256 after the letter C. The default value is zero. Alternatively, the argument may be a delimited string. The starting delimiter must be one of ` ' " ^ % # $ { and the ending delimiter is the same as the start, except for {, where the end- ing delimiter is }. If the ending delimiter is needed within the string, it must be doubled. For example, this pattern has two callout points:
(?C1)abc(?C"some ""arbitrary"" text")def
If the PCRE2_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE2 automatically inserts callouts, all with number 255, before each item in the pattern except for immediately before or after an explicit callout. For example, if PCRE2_AUTO_CALLOUT is used with the pattern
A(?C3)B
it is processed as if it were
(?C255)A(?C3)B(?C255)
Here is a more complicated example:
A(\d{2}|--)
With PCRE2_AUTO_CALLOUT, this pattern is processed as if it were
(?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
Notice that there is a callout before and after each parenthesis and alternation bar. If the pattern contains a conditional group whose con- dition is an assertion, an automatic callout is inserted immediately before the condition. Such a callout may also be inserted explicitly, for example:
(?(?C9)(?=a)ab|de) (?(?C%text%)(?!=d)ab|de)
This applies only to assertion conditions (because they are themselves independent groups).
Callouts can be useful for tracking the progress of pattern matching. The pcre2test program has a pattern qualifier (/auto_callout) that sets automatic callouts. When any callouts are present, the output from pcre2test indicates how the pattern is being matched. This is useful information when you are trying to optimize the performance of a par- ticular pattern.
MISSING CALLOUTS
You should be aware that, because of optimizations in the way PCRE2 compiles and matches patterns, callouts sometimes do not happen exactly as you might expect.
Auto-possessification
At compile time, PCRE2 "auto-possessifies" repeated items when it knows that what follows cannot be part of the repeat. For example, a+[bc] is compiled as if it were a++[bc]. The pcre2test output when this pattern is compiled with PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied to the string "aaaa" is:
--->aaaa +0 ^ a+ +2 ^ ^ [bc] No match
This indicates that when matching [bc] fails, there is no backtracking into a+ (because it is being treated as a++) and therefore the callouts that would be taken for the backtracks do not occur. You can disable the auto-possessify feature by passing PCRE2_NO_AUTO_POSSESS to pcre2_compile(), or starting the pattern with (*NO_AUTO_POSSESS). In this case, the output changes to this:
--->aaaa +0 ^ a+ +2 ^ ^ [bc] +2 ^ ^ [bc] +2 ^ ^ [bc] +2 ^^ [bc] No match
This time, when matching [bc] fails, the matcher backtracks into a+ and tries again, repeatedly, until a+ itself fails.
Automatic .* anchoring
By default, an optimization is applied when .* is the first significant item in a pattern. If PCRE2_DOTALL is set, so that the dot can match any character, the pattern is automatically anchored. If PCRE2_DOTALL is not set, a match can start only after an internal newline or at the beginning of the subject, and pcre2_compile() remembers this. If a pat- tern has more than one top-level branch, automatic anchoring occurs if all branches are anchorable.
This optimization is disabled, however, if .* is in an atomic group or if there is a backreference to the capture group in which it appears. It is also disabled if the pattern contains (*PRUNE) or (*SKIP). How- ever, the presence of callouts does not affect it.
For example, if the pattern .*\d is compiled with PCRE2_AUTO_CALLOUT and applied to the string "aa", the pcre2test output is:
--->aa +0 ^ .* +2 ^ ^ \d +2 ^^ \d +2 ^ \d No match
This shows that all match attempts start at the beginning of the sub- ject. In other words, the pattern is anchored. You can disable this op- timization by passing PCRE2_NO_DOTSTAR_ANCHOR to pcre2_compile(), or starting the pattern with (*NO_DOTSTAR_ANCHOR). In this case, the out- put changes to:
--->aa +0 ^ .* +2 ^ ^ \d +2 ^^ \d +2 ^ \d +0 ^ .* +2 ^^ \d +2 ^ \d No match
This shows more match attempts, starting at the second subject charac- ter. Another optimization, described in the next section, means that there is no subsequent attempt to match with an empty subject.
Other optimizations
Other optimizations that provide fast "no match" results also affect callouts. For example, if the pattern is
ab(?C4)cd
PCRE2 knows that any matching string must contain the letter "d". If the subject string is "abyz", the lack of "d" means that matching doesn't ever start, and the callout is never reached. However, with "abyd", though the result is still no match, the callout is obeyed.
For most patterns PCRE2 also knows the minimum length of a matching string, and will immediately give a "no match" return without actually running a match if the subject is not long enough, or, for unanchored patterns, if it has been scanned far enough.
You can disable these optimizations by passing the PCRE2_NO_START_OPTI- MIZE option to pcre2_compile(), or by starting the pattern with (*NO_START_OPT). This slows down the matching process, but does ensure that callouts such as the example above are obeyed.
THE CALLOUT INTERFACE
During matching, when PCRE2 reaches a callout point, if an external function is provided in the match context, it is called. This applies to both normal, DFA, and JIT matching. The first argument to the call- out function is a pointer to a pcre2_callout block. The second argument is the void * callout data that was supplied when the callout was set up by calling pcre2_set_callout() (see the pcre2api documentation). The callout block structure contains the following fields, not necessarily in this order:
uint32_t version; uint32_t callout_number; uint32_t capture_top; uint32_t capture_last; uint32_t callout_flags; PCRE2_SIZE *offset_vector; PCRE2_SPTR mark; PCRE2_SPTR subject; PCRE2_SIZE subject_length; PCRE2_SIZE start_match; PCRE2_SIZE current_position; PCRE2_SIZE pattern_position; PCRE2_SIZE next_item_length; PCRE2_SIZE callout_string_offset; PCRE2_SIZE callout_string_length; PCRE2_SPTR callout_string;
The version field contains the version number of the block format. The current version is 2; the three callout string fields were added for version 1, and the callout_flags field for version 2. If you are writ- ing an application that might use an earlier release of PCRE2, you should check the version number before accessing any of these fields. The version number will increase in future if more fields are added, but the intention is never to remove any of the existing fields.
Fields for numerical callouts
For a numerical callout, callout_string is NULL, and callout_number contains the number of the callout, in the range 0-255. This is the number that follows (?C for callouts that part of the pattern; it is 255 for automatically generated callouts.
Fields for string callouts
For callouts with string arguments, callout_number is always zero, and callout_string points to the string that is contained within the com- piled pattern. Its length is given by callout_string_length. Duplicated ending delimiters that were present in the original pattern string have been turned into single characters, but there is no other processing of the callout string argument. An additional code unit containing binary zero is present after the string, but is not included in the length. The delimiter that was used to start the string is also stored within the pattern, immediately before the string itself. You can access this delimiter as callout_string[-1] if you need it.
The callout_string_offset field is the code unit offset to the start of the callout argument string within the original pattern string. This is provided for the benefit of applications such as script languages that might need to report errors in the callout string within the pattern.
Fields for all callouts
The remaining fields in the callout block are the same for both kinds of callout.
The offset_vector field is a pointer to a vector of capturing offsets (the "ovector"). You may read the elements in this vector, but you must not change any of them.
For calls to pcre2_match(), the offset_vector field is not (since re- lease 10.30) a pointer to the actual ovector that was passed to the matching function in the match data block. Instead it points to an in- ternal ovector of a size large enough to hold all possible captured substrings in the pattern. Note that whenever a recursion or subroutine call within a pattern completes, the capturing state is reset to what it was before.
The capture_last field contains the number of the most recently cap- tured substring, and the capture_top field contains one more than the number of the highest numbered captured substring so far. If no sub- strings have yet been captured, the value of capture_last is 0 and the value of capture_top is 1. The values of these fields do not always differ by one; for example, when the callout in the pattern ((a)(b))(?C2) is taken, capture_last is 1 but capture_top is 4.
The contents of ovector[2] to ovector[<capture_top>*2-1] can be in- spected in order to extract substrings that have been matched so far, in the same way as extracting substrings after a match has completed. The values in ovector[0] and ovector[1] are always PCRE2_UNSET because the match is by definition not complete. Substrings that have not been captured but whose numbers are less than capture_top also have both of their ovector slots set to PCRE2_UNSET.
For DFA matching, the offset_vector field points to the ovector that was passed to the matching function in the match data block for call- outs at the top level, but to an internal ovector during the processing of pattern recursions, lookarounds, and atomic groups. However, these ovectors hold no useful information because pcre2_dfa_match() does not support substring capturing. The value of capture_top is always 1 and the value of capture_last is always 0 for DFA matching.
The subject and subject_length fields contain copies of the values that were passed to the matching function.
The start_match field normally contains the offset within the subject at which the current match attempt started. However, if the escape se- quence \K has been encountered, this value is changed to reflect the modified starting point. If the pattern is not anchored, the callout function may be called several times from the same point in the pattern for different starting points in the subject.
The current_position field contains the offset within the subject of the current match pointer.
The pattern_position field contains the offset in the pattern string to the next item to be matched.
The next_item_length field contains the length of the next item to be processed in the pattern string. When the callout is at the end of the pattern, the length is zero. When the callout precedes an opening parenthesis, the length includes meta characters that follow the paren- thesis. For example, in a callout before an assertion such as (?=ab) the length is 3. For an alternation bar or a closing parenthesis, the length is one, unless a closing parenthesis is followed by a quanti- fier, in which case its length is included. (This changed in release 10.23. In earlier releases, before an opening parenthesis the length was that of the entire group, and before an alternation bar or a clos- ing parenthesis the length was zero.)
The pattern_position and next_item_length fields are intended to help in distinguishing between different automatic callouts, which all have the same callout number. However, they are set for all callouts, and are used by pcre2test to show the next item to be matched when display- ing callout information.
In callouts from pcre2_match() the mark field contains a pointer to the zero-terminated name of the most recently passed (*MARK), (*PRUNE), or (*THEN) item in the match, or NULL if no such items have been passed. Instances of (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In callouts from the DFA matching function this field always contains NULL.
The callout_flags field is always zero in callouts from pcre2_dfa_match() or when JIT is being used. When pcre2_match() without JIT is used, the following bits may be set:
PCRE2_CALLOUT_STARTMATCH
This is set for the first callout after the start of matching for each new starting position in the subject.
PCRE2_CALLOUT_BACKTRACK
This is set if there has been a matching backtrack since the previous callout, or since the start of matching if this is the first callout from a pcre2_match() run.
Both bits are set when a backtrack has caused a "bumpalong" to a new starting position in the subject. Output from pcre2test does not indi- cate the presence of these bits unless the callout_extra modifier is set.
The information in the callout_flags field is provided so that applica- tions can track and tell their users how matching with backtracking is done. This can be useful when trying to optimize patterns, or just to understand how PCRE2 works. There is no support in pcre2_dfa_match() because there is no backtracking in DFA matching, and there is no sup- port in JIT because JIT is all about maximimizing matching performance. In both these cases the callout_flags field is always zero.
RETURN VALUES FROM CALLOUTS
The external callout function returns an integer to PCRE2. If the value is zero, matching proceeds as normal. If the value is greater than zero, matching fails at the current point, but the testing of other matching possibilities goes ahead, just as if a lookahead assertion had failed. If the value is less than zero, the match is abandoned, and the matching function returns the negative value.
Negative values should normally be chosen from the set of PCRE2_ER- ROR_xxx values. In particular, PCRE2_ERROR_NOMATCH forces a standard "no match" failure. The error number PCRE2_ERROR_CALLOUT is reserved for use by callout functions; it will never be used by PCRE2 itself.
CALLOUT ENUMERATION
int pcre2_callout_enumerate(const pcre2_code *code, int (*callback)(pcre2_callout_enumerate_block *, void *), void *user_data);
A script language that supports the use of string arguments in callouts might like to scan all the callouts in a pattern before running the match. This can be done by calling pcre2_callout_enumerate(). The first argument is a pointer to a compiled pattern, the second points to a callback function, and the third is arbitrary user data. The callback function is called for every callout in the pattern in the order in which they appear. Its first argument is a pointer to a callout enumer- ation block, and its second argument is the user_data value that was passed to pcre2_callout_enumerate(). The data block contains the fol- lowing fields:
version Block version number pattern_position Offset to next item in pattern next_item_length Length of next item in pattern callout_number Number for numbered callouts callout_string_offset Offset to string within pattern callout_string_length Length of callout string callout_string Points to callout string or is NULL
The version number is currently 0. It will increase if new fields are ever added to the block. The remaining fields are the same as their namesakes in the pcre2_callout block that is used for callouts during matching, as described above.
Note that the value of pattern_position is unique for each callout. However, if a callout occurs inside a group that is quantified with a non-zero minimum or a fixed maximum, the group is replicated inside the compiled pattern. For example, a pattern such as /(a){2}/ is compiled as if it were /(a)(a)/. This means that the callout will be enumerated more than once, but with the same value for pattern_position in each case.
The callback function should normally return zero. If it returns a non- zero value, scanning the pattern stops, and that value is returned from pcre2_callout_enumerate().
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
REVISION
Last updated: 19 January 2024 Copyright (c) 1997-2024 University of Cambridge.
PCRE2 10.43 19 January 2024 PCRE2CALLOUT(3)------------------------------------------------------------------------------
PCRE2COMPAT(3) Library Functions Manual PCRE2COMPAT(3)
NAME PCRE2 - Perl-compatible regular expressions (revised API)
DIFFERENCES BETWEEN PCRE2 AND PERL
This document describes some of the known differences in the ways that PCRE2 and Perl handle regular expressions. The differences described here are with respect to Perl version 5.38.0, but as both Perl and PCRE2 are continually changing, the information may at times be out of date.
1. When PCRE2_DOTALL (equivalent to Perl's /s qualifier) is not set, the behaviour of the '.' metacharacter differs from Perl. In PCRE2, '.' matches the next character unless it is the start of a newline se- quence. This means that, if the newline setting is CR, CRLF, or NUL, '.' will match the code point LF (0x0A) in ASCII/Unicode environments, and NL (either 0x15 or 0x25) when using EBCDIC. In Perl, '.' appears never to match LF, even when 0x0A is not a newline indicator.
2. PCRE2 has only a subset of Perl's Unicode support. Details of what it does have are given in the pcre2unicode page.
3. Like Perl, PCRE2 allows repeat quantifiers on parenthesized asser- tions, but they do not mean what you might think. For example, (?!a){3} does not assert that the next three characters are not "a". It just as- serts that the next character is not "a" three times (in principle; PCRE2 optimizes this to run the assertion just once). Perl allows some repeat quantifiers on other assertions, for example, \b* , but these do not seem to have any use. PCRE2 does not allow any kind of quantifier on non-lookaround assertions.
4. If a braced quantifier such as {1,2} appears where there is nothing to repeat (for example, at the start of a branch), PCRE2 raises an er- ror whereas Perl treats the quantifier characters as literal.
5. Capture groups that occur inside negative lookaround assertions are counted, but their entries in the offsets vector are set only when a negative assertion is a condition that has a matching branch (that is, the condition is false). Perl may set such capture groups in other circumstances.
6. The following Perl escape sequences are not supported: \F, \l, \L, \u, \U, and \N when followed by a character name. \N on its own, match- ing a non-newline character, and \N{U+dd..}, matching a Unicode code point, are supported. The escapes that modify the case of following letters are implemented by Perl's general string-handling and are not part of its pattern matching engine. If any of these are encountered by PCRE2, an error is generated by default. However, if either of the PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX options is set, \U and \u are interpreted as ECMAScript interprets them.
7. The Perl escape sequences \p, \P, and \X are supported only if PCRE2 is built with Unicode support (the default). The properties that can be tested with \p and \P are limited to the general category properties such as Lu and Nd, the derived properties Any and LC (synonym L&), script names such as Greek or Han, Bidi_Class, Bidi_Control, and a few binary properties. Both PCRE2 and Perl support the Cs (surrogate) prop- erty, but in PCRE2 its use is limited. See the pcre2pattern documenta- tion for details. The long synonyms for property names that Perl sup- ports (such as \p{Letter}) are not supported by PCRE2, nor is it per- mitted to prefix any of these properties with "Is".
8. PCRE2 supports the \Q...\E escape for quoting substrings. Characters in between are treated as literals. However, this is slightly different from Perl in that $ and @ are also handled as literals inside the quotes. In Perl, they cause variable interpolation (PCRE2 does not have variables). Also, Perl does "double-quotish backslash interpolation" on any backslashes between \Q and \E which, its documentation says, "may lead to confusing results". PCRE2 treats a backslash between \Q and \E just like any other character. Note the following examples:
Pattern PCRE2 matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the contents of $xyz \Qabc\$xyz\E abc\$xyz abc\$xyz \Qabc\E\$\Qxyz\E abc$xyz abc$xyz \QA\B\E A\B A\B \Q\\E \ \\E
The \Q...\E sequence is recognized both inside and outside character classes by both PCRE2 and Perl.
9. Fairly obviously, PCRE2 does not support the (?{code}) and (??{code}) constructions. However, PCRE2 does have a "callout" feature, which allows an external function to be called during pattern matching. See the pcre2callout documentation for details.
10. Subroutine calls (whether recursive or not) were treated as atomic groups up to PCRE2 release 10.23, but from release 10.30 this changed, and backtracking into subroutine calls is now supported, as in Perl.
11. In PCRE2, if any of the backtracking control verbs are used in a group that is called as a subroutine (whether or not recursively), their effect is confined to that group; it does not extend to the sur- rounding pattern. This is not always the case in Perl. In particular, if (*THEN) is present in a group that is called as a subroutine, its action is limited to that group, even if the group does not contain any | characters. Note that such groups are processed as anchored at the point where they are tested.
12. If a pattern contains more than one backtracking control verb, the first one that is backtracked onto acts. For example, in the pattern A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the same as PCRE2, but there are cases where it differs.
13. There are some differences that are concerned with the settings of captured strings when part of a pattern is repeated. For example, matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 un- set, but in PCRE2 it is set to "b".
14. PCRE2's handling of duplicate capture group numbers and names is not as general as Perl's. This is a consequence of the fact the PCRE2 works internally just with numbers, using an external table to trans- late between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b>B)), where the two capture groups have the same number but different names, is not supported, and causes an error at compile time. If it were allowed, it would not be possible to distinguish which group matched, because both names map to capture group number 1. To avoid this confusing situation, an error is given at compile time.
15. Perl used to recognize comments in some places that PCRE2 does not, for example, between the ( and ? at the start of a group. If the /x modifier is set, Perl allowed white space between ( and ? though the latest Perls give an error (for a while it was just deprecated). There may still be some cases where Perl behaves differently.
16. Perl, when in warning mode, gives warnings for character classes such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as liter- als. PCRE2 has no warning features, so it gives an error in these cases because they are almost certainly user mistakes.
17. In PCRE2, the upper/lower case character properties Lu and Ll are not affected when case-independent matching is specified. For example, \p{Lu} always matches an upper case letter. I think Perl has changed in this respect; in the release at the time of writing (5.38), \p{Lu} and \p{Ll} match all letters, regardless of case, when case independence is specified.
18. From release 5.32.0, Perl locks out the use of \K in lookaround as- sertions. From release 10.38 PCRE2 does the same by default. However, there is an option for re-enabling the previous behaviour. When this option is set, \K is acted on when it occurs in positive assertions, but is ignored in negative assertions.
19. PCRE2 provides some extensions to the Perl regular expression fa- cilities. Perl 5.10 included new features that were not in earlier versions of Perl, some of which (such as named parentheses) were in PCRE2 for some time before. This list is with respect to Perl 5.38:
(a) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the $ meta-character matches only at the very end of the string.
(b) A backslash followed by a letter with no special meaning is faulted. (Perl can be made to issue a warning.)
(c) If PCRE2_UNGREEDY is set, the greediness of the repetition quanti- fiers is inverted, that is, by default they are not greedy, but if fol- lowed by a question mark they are.
(d) PCRE2_ANCHORED can be used at matching time to force a pattern to be tried only at the first matching position in the subject string.
(e) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents.
(f) The \R escape sequence can be restricted to match only CR, LF, or CRLF by the PCRE2_BSR_ANYCRLF option.
(g) The callout facility is PCRE2-specific. Perl supports codeblocks and variable interpolation, but not general hooks on every match.
(h) The partial matching facility is PCRE2-specific.
(i) The alternative matching function (pcre2_dfa_match() matches in a different way and is not Perl-compatible.
(j) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT) at the start of a pattern. These set overall options that cannot be changed within the pattern.
(k) PCRE2 supports non-atomic positive lookaround assertions. This is an extension to the lookaround facilities. The default, Perl-compatible lookarounds are atomic.
(l) There are three syntactical items in patterns that can refer to a capturing group by number: back references such as \g{2}, subroutine calls such as (?3), and condition references such as (?(4)...). PCRE2 supports relative group numbers such as +2 and -4 in all three cases. Perl supports both plus and minus for subroutine calls, but only minus for back references, and no relative numbering at all for conditions.
20. Perl has different limits than PCRE2. See the pcre2limit documenta- tion for details. Perl went with 5.10 from recursion to iteration keep- ing the intermediate matches on the heap, which is ~10% slower but does not fall into any stack-overflow limit. PCRE2 made a similar change at release 10.30, and also has many build-time and run-time customizable limits.
21. Unlike Perl, PCRE2 doesn't have character set modifiers and spe- cially no way to set characters by context just like Perl's "/d". A regular expression using PCRE2_UTF and PCRE2_UCP will use similar rules to Perl's "/u"; something closer to "/a" could be selected by adding other PCRE2_EXTRA_ASCII* options on top.
22. Some recursive patterns that Perl diagnoses as infinite recursions can be handled by PCRE2, either by the interpreter or the JIT. An exam- ple is /(?:|(?0)abcd)(?(R)|\z)/, which matches a sequence of any number of repeated "abcd" substrings at the end of the subject.
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
REVISION
Last updated: 30 November 2023 Copyright (c) 1997-2023 University of Cambridge.
PCRE2 10.43 30 November 2023 PCRE2COMPAT(3)------------------------------------------------------------------------------
PCRE2JIT(3) Library Functions Manual PCRE2JIT(3)
NAME PCRE2 - Perl-compatible regular expressions (revised API)
PCRE2 JUST-IN-TIME COMPILER SUPPORT
Just-in-time compiling is a heavyweight optimization that can greatly speed up pattern matching. However, it comes at the cost of extra pro- cessing before the match is performed, so it is of most benefit when the same pattern is going to be matched many times. This does not nec- essarily mean many calls of a matching function; if the pattern is not anchored, matching attempts may take place many times at various posi- tions in the subject, even for a single call. Therefore, if the subject string is very long, it may still pay to use JIT even for one-off matches. JIT support is available for all of the 8-bit, 16-bit and 32-bit PCRE2 libraries.
JIT support applies only to the traditional Perl-compatible matching function. It does not apply when the DFA matching function is being used. The code for JIT support was written by Zoltan Herczeg.
AVAILABILITY OF JIT SUPPORT
JIT support is an optional feature of PCRE2. The "configure" option --enable-jit (or equivalent CMake option) must be set when PCRE2 is built if you want to use JIT. The support is limited to the following hardware platforms:
ARM 32-bit (v7, and Thumb2) ARM 64-bit IBM s390x 64 bit Intel x86 32-bit and 64-bit LoongArch 64 bit MIPS 32-bit and 64-bit Power PC 32-bit and 64-bit RISC-V 32-bit and 64-bit
If --enable-jit is set on an unsupported platform, compilation fails.
A client program can tell if JIT support is available by calling pcre2_config() with the PCRE2_CONFIG_JIT option. The result is one if PCRE2 was built with JIT support, and zero otherwise. However, having the JIT code available does not guarantee that it will be used for any particular match. One reason for this is that there are a number of op- tions and pattern items that are not supported by JIT (see below). An- other reason is that in some environments JIT is unable to get memory in which to build its compiled code. The only guarantee from pcre2_con- fig() is that if it returns zero, JIT will definitely not be used.
A simple program does not need to check availability in order to use JIT when possible. The API is implemented in a way that falls back to the interpretive code if JIT is not available or cannot be used for a given match. For programs that need the best possible performance, there is a "fast path" API that is JIT-specific.
SIMPLE USE OF JIT
To make use of the JIT support in the simplest way, all you have to do is to call pcre2_jit_compile() after successfully compiling a pattern with pcre2_compile(). This function has two arguments: the first is the compiled pattern pointer that was returned by pcre2_compile(), and the second is zero or more of the following option bits: PCRE2_JIT_COM- PLETE, PCRE2_JIT_PARTIAL_HARD, or PCRE2_JIT_PARTIAL_SOFT.
If JIT support is not available, a call to pcre2_jit_compile() does nothing and returns PCRE2_ERROR_JIT_BADOPTION. Otherwise, the compiled pattern is passed to the JIT compiler, which turns it into machine code that executes much faster than the normal interpretive code, but yields exactly the same results. The returned value from pcre2_jit_compile() is zero on success, or a negative error code.
There is a limit to the size of pattern that JIT supports, imposed by the size of machine stack that it uses. The exact rules are not docu- mented because they may change at any time, in particular, when new op- timizations are introduced. If a pattern is too big, a call to pcre2_jit_compile() returns PCRE2_ERROR_NOMEMORY.
PCRE2_JIT_COMPLETE requests the JIT compiler to generate code for com- plete matches. If you want to run partial matches using the PCRE2_PAR- TIAL_HARD or PCRE2_PARTIAL_SOFT options of pcre2_match(), you should set one or both of the other options as well as, or instead of PCRE2_JIT_COMPLETE. The JIT compiler generates different optimized code for each of the three modes (normal, soft partial, hard partial). When pcre2_match() is called, the appropriate code is run if it is avail- able. Otherwise, the pattern is matched using interpretive code.
You can call pcre2_jit_compile() multiple times for the same compiled pattern. It does nothing if it has previously compiled code for any of the option bits. For example, you can call it once with PCRE2_JIT_COM- PLETE and (perhaps later, when you find you need partial matching) again with PCRE2_JIT_COMPLETE and PCRE2_JIT_PARTIAL_HARD. This time it will ignore PCRE2_JIT_COMPLETE and just compile code for partial match- ing. If pcre2_jit_compile() is called with no option bits set, it imme- diately returns zero. This is an alternative way of testing whether JIT is available.
At present, it is not possible to free JIT compiled code except when the entire compiled pattern is freed by calling pcre2_code_free().
In some circumstances you may need to call additional functions. These are described in the section entitled "Controlling the JIT stack" be- low.
There are some pcre2_match() options that are not supported by JIT, and there are also some pattern items that JIT cannot handle. Details are given below. In both cases, matching automatically falls back to the interpretive code. If you want to know whether JIT was actually used for a particular match, you should arrange for a JIT callback function to be set up as described in the section entitled "Controlling the JIT stack" below, even if you do not need to supply a non-default JIT stack. Such a callback function is called whenever JIT code is about to be obeyed. If the match-time options are not right for JIT execution, the callback function is not obeyed.
If the JIT compiler finds an unsupported item, no JIT data is gener- ated. You can find out if JIT compilation was successful for a compiled pattern by calling pcre2_pattern_info() with the PCRE2_INFO_JITSIZE op- tion. A non-zero result means that JIT compilation was successful. A result of 0 means that JIT support is not available, or the pattern was not processed by pcre2_jit_compile(), or the JIT compiler was not able to handle the pattern. Successful JIT compilation does not, however, guarantee the use of JIT at match time because there are some match time options that are not supported by JIT.
MATCHING SUBJECTS CONTAINING INVALID UTF
When a pattern is compiled with the PCRE2_UTF option, subject strings are normally expected to be a valid sequence of UTF code units. By de- fault, this is checked at the start of matching and an error is gener- ated if invalid UTF is detected. The PCRE2_NO_UTF_CHECK option can be passed to pcre2_match() to skip the check (for improved performance) if you are sure that a subject string is valid. If this option is used with an invalid string, the result is undefined. The calling program may crash or loop or otherwise misbehave.
However, a way of running matches on strings that may contain invalid UTF sequences is available. Calling pcre2_compile() with the PCRE2_MATCH_INVALID_UTF option has two effects: it tells the inter- preter in pcre2_match() to support invalid UTF, and, if pcre2_jit_com- pile() is subsequently called, the compiled JIT code also supports in- valid UTF. Details of how this support works, in both the JIT and the interpretive cases, is given in the pcre2unicode documentation.
There is also an obsolete option for pcre2_jit_compile() called PCRE2_JIT_INVALID_UTF, which currently exists only for backward compat- ibility. It is superseded by the pcre2_compile() option PCRE2_MATCH_INVALID_UTF and should no longer be used. It may be removed in future.
UNSUPPORTED OPTIONS AND PATTERN ITEMS
The pcre2_match() options that are supported for JIT matching are PCRE2_COPY_MATCHED_SUBJECT, PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and PCRE2_PARTIAL_SOFT. The PCRE2_ANCHORED and PCRE2_ENDANCHORED options are not supported at match time.
If the PCRE2_NO_JIT option is passed to pcre2_match() it disables the use of JIT, forcing matching by the interpreter code.
The only unsupported pattern items are \C (match a single data unit) when running in a UTF mode, and a callout immediately before an asser- tion condition in a conditional group.
RETURN VALUES FROM JIT MATCHING
When a pattern is matched using JIT, the return values are the same as those given by the interpretive pcre2_match() code, with the addition of one new error code: PCRE2_ERROR_JIT_STACKLIMIT. This means that the memory used for the JIT stack was insufficient. See "Controlling the JIT stack" below for a discussion of JIT stack usage.
The error code PCRE2_ERROR_MATCHLIMIT is returned by the JIT code if searching a very large pattern tree goes on for too long, as it is in the same circumstance when JIT is not used, but the details of exactly what is counted are not the same. The PCRE2_ERROR_DEPTHLIMIT error code is never returned when JIT matching is used.
CONTROLLING THE JIT STACK
When the compiled JIT code runs, it needs a block of memory to use as a stack. By default, it uses 32KiB on the machine stack. However, some large or complicated patterns need more than this. The error PCRE2_ER- ROR_JIT_STACKLIMIT is given when there is not enough stack. Three func- tions are provided for managing blocks of memory for use as JIT stacks. There is further discussion about the use of JIT stacks in the section entitled "JIT stack FAQ" below.
The pcre2_jit_stack_create() function creates a JIT stack. Its argu- ments are a starting size, a maximum size, and a general context (for memory allocation functions, or NULL for standard memory allocation). It returns a pointer to an opaque structure of type pcre2_jit_stack, or NULL if there is an error. The pcre2_jit_stack_free() function is used to free a stack that is no longer needed. If its argument is NULL, this function returns immediately, without doing anything. (For the techni- cally minded: the address space is allocated by mmap or VirtualAlloc.) A maximum stack size of 512KiB to 1MiB should be more than enough for any pattern.
The pcre2_jit_stack_assign() function specifies which stack JIT code should use. Its arguments are as follows:
pcre2_match_context *mcontext pcre2_jit_callback callback void *data
The first argument is a pointer to a match context. When this is subse- quently passed to a matching function, its information determines which JIT stack is used. If this argument is NULL, the function returns imme- diately, without doing anything. There are three cases for the values of the other two options:
(1) If callback is NULL and data is NULL, an internal 32KiB block on the machine stack is used. This is the default when a match context is created.
(2) If callback is NULL and data is not NULL, data must be a pointer to a valid JIT stack, the result of calling pcre2_jit_stack_create().
(3) If callback is not NULL, it must point to a function that is called with data as an argument at the start of matching, in order to set up a JIT stack. If the return from the callback function is NULL, the internal 32KiB stack is used; otherwise the return value must be a valid JIT stack, the result of calling pcre2_jit_stack_create().
A callback function is obeyed whenever JIT code is about to be run; it is not obeyed when pcre2_match() is called with options that are incom- patible for JIT matching. A callback function can therefore be used to determine whether a match operation was executed by JIT or by the in- terpreter.
You may safely use the same JIT stack for more than one pattern (either by assigning directly or by callback), as long as the patterns are matched sequentially in the same thread. Currently, the only way to set up non-sequential matches in one thread is to use callouts: if a call- out function starts another match, that match must use a different JIT stack to the one used for currently suspended match(es).
In a multithread application, if you do not specify a JIT stack, or if you assign or pass back NULL from a callback, that is thread-safe, be- cause each thread has its own machine stack. However, if you assign or pass back a non-NULL JIT stack, this must be a different stack for each thread so that the application is thread-safe.
Strictly speaking, even more is allowed. You can assign the same non- NULL stack to a match context that is used by any number of patterns, as long as they are not used for matching by multiple threads at the same time. For example, you could use the same stack in all compiled patterns, with a global mutex in the callback to wait until the stack is available for use. However, this is an inefficient solution, and not recommended.
This is a suggestion for how a multithreaded program that needs to set up non-default JIT stacks might operate:
During thread initialization thread_local_var = pcre2_jit_stack_create(...)
During thread exit pcre2_jit_stack_free(thread_local_var)
Use a one-line callback function return thread_local_var
All the functions described in this section do nothing if JIT is not available.
JIT STACK FAQ
(1) Why do we need JIT stacks?
PCRE2 (and JIT) is a recursive, depth-first engine, so it needs a stack where the local data of the current node is pushed before checking its child nodes. Allocating real machine stack on some platforms is diffi- cult. For example, the stack chain needs to be updated every time if we extend the stack on PowerPC. Although it is possible, its updating time overhead decreases performance. So we do the recursion in memory.
(2) Why don't we simply allocate blocks of memory with malloc()?
Modern operating systems have a nice feature: they can reserve an ad- dress space instead of allocating memory. We can safely allocate memory pages inside this address space, so the stack could grow without moving memory data (this is important because of pointers). Thus we can allo- cate 1MiB address space, and use only a single memory page (usually 4KiB) if that is enough. However, we can still grow up to 1MiB anytime if needed.
(3) Who "owns" a JIT stack?
The owner of the stack is the user program, not the JIT studied pattern or anything else. The user program must ensure that if a stack is being used by pcre2_match(), (that is, it is assigned to a match context that is passed to the pattern currently running), that stack must not be used by any other threads (to avoid overwriting the same memory area). The best practice for multithreaded programs is to allocate a stack for each thread, and return this stack through the JIT callback function.
(4) When should a JIT stack be freed?
You can free a JIT stack at any time, as long as it will not be used by pcre2_match() again. When you assign the stack to a match context, only a pointer is set. There is no reference counting or any other magic. You can free compiled patterns, contexts, and stacks in any order, any- time. Just do not call pcre2_match() with a match context pointing to an already freed stack, as that will cause SEGFAULT. (Also, do not free a stack currently used by pcre2_match() in another thread). You can also replace the stack in a context at any time when it is not in use. You should free the previous stack before assigning a replacement.
(5) Should I allocate/free a stack every time before/after calling pcre2_match()?
No, because this is too costly in terms of resources. However, you could implement some clever idea which release the stack if it is not used in let's say two minutes. The JIT callback can help to achieve this without keeping a list of patterns.
(6) OK, the stack is for long term memory allocation. But what happens if a pattern causes stack overflow with a stack of 1MiB? Is that 1MiB kept until the stack is freed?
Especially on embedded systems, it might be a good idea to release mem- ory sometimes without freeing the stack. There is no API for this at the moment. Probably a function call which returns with the currently allocated memory for any stack and another which allows releasing mem- ory (shrinking the stack) would be a good idea if someone needs this.
(7) This is too much of a headache. Isn't there any better solution for JIT stack handling?
No, thanks to Windows. If POSIX threads were used everywhere, we could throw out this complicated API.
FREEING JIT SPECULATIVE MEMORY
void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);
The JIT executable allocator does not free all memory when it is possi- ble. It expects new allocations, and keeps some free memory around to improve allocation speed. However, in low memory conditions, it might be better to free all possible memory. You can cause this to happen by calling pcre2_jit_free_unused_memory(). Its argument is a general con- text, for custom memory management, or NULL for standard memory manage- ment.
EXAMPLE CODE
This is a single-threaded example that specifies a JIT stack without using a callback. A real program should include error checking after all the function calls.
int rc; pcre2_code *re; pcre2_match_data *match_data; pcre2_match_context *mcontext; pcre2_jit_stack *jit_stack;
re = pcre2_compile(pattern, PCRE2_ZERO_TERMINATED, 0, &errornumber, &erroffset, NULL); rc = pcre2_jit_compile(re, PCRE2_JIT_COMPLETE); mcontext = pcre2_match_context_create(NULL); jit_stack = pcre2_jit_stack_create(32*1024, 512*1024, NULL); pcre2_jit_stack_assign(mcontext, NULL, jit_stack); match_data = pcre2_match_data_create(re, 10); rc = pcre2_match(re, subject, length, 0, 0, match_data, mcontext); /* Process result */
pcre2_code_free(re); pcre2_match_data_free(match_data); pcre2_match_context_free(mcontext); pcre2_jit_stack_free(jit_stack);
JIT FAST PATH API
Because the API described above falls back to interpreted matching when JIT is not available, it is convenient for programs that are written for general use in many environments. However, calling JIT via pcre2_match() does have a performance impact. Programs that are written for use where JIT is known to be available, and which need the best possible performance, can instead use a "fast path" API to call JIT matching directly instead of calling pcre2_match() (obviously only for patterns that have been successfully processed by pcre2_jit_compile()).
The fast path function is called pcre2_jit_match(), and it takes ex- actly the same arguments as pcre2_match(). However, the subject string must be specified with a length; PCRE2_ZERO_TERMINATED is not sup- ported. Unsupported option bits (for example, PCRE2_ANCHORED and PCRE2_ENDANCHORED) are ignored, as is the PCRE2_NO_JIT option. The re- turn values are also the same as for pcre2_match(), plus PCRE2_ER- ROR_JIT_BADOPTION if a matching mode (partial or complete) is requested that was not compiled.
When you call pcre2_match(), as well as testing for invalid options, a number of other sanity checks are performed on the arguments. For exam- ple, if the subject pointer is NULL but the length is non-zero, an im- mediate error is given. Also, unless PCRE2_NO_UTF_CHECK is set, a UTF subject string is tested for validity. In the interests of speed, these checks do not happen on the JIT fast path. If invalid UTF data is passed when PCRE2_MATCH_INVALID_UTF was not set for pcre2_compile(), the result is undefined. The program may crash or loop or give wrong results. In the absence of PCRE2_MATCH_INVALID_UTF you should call pcre2_jit_match() in UTF mode only if you are sure the subject is valid.
Bypassing the sanity checks and the pcre2_match() wrapping can give speedups of more than 10%.
SEE ALSO
pcre2api(3), pcre2unicode(3)
AUTHOR
Philip Hazel (FAQ by Zoltan Herczeg) Retired from University Computing Service Cambridge, England.
REVISION
Last updated: 21 February 2024 Copyright (c) 1997-2024 University of Cambridge.
PCRE2 10.43 21 February 2024 PCRE2JIT(3)------------------------------------------------------------------------------
PCRE2LIMITS(3) Library Functions Manual PCRE2LIMITS(3)
NAME PCRE2 - Perl-compatible regular expressions (revised API)
SIZE AND OTHER LIMITATIONS
There are some size limitations in PCRE2 but it is hoped that they will never in practice be relevant.
The maximum size of a compiled pattern is approximately 64 thousand code units for the 8-bit and 16-bit libraries if PCRE2 is compiled with the default internal linkage size, which is 2 bytes for these li- braries. If you want to process regular expressions that are truly enormous, you can compile PCRE2 with an internal linkage size of 3 or 4 (when building the 16-bit library, 3 is rounded up to 4). See the README file in the source distribution and the pcre2build documentation for details. In these cases the limit is substantially larger. How- ever, the speed of execution is slower. In the 32-bit library, the in- ternal linkage size is always 4.
The maximum length of a source pattern string is essentially unlimited; it is the largest number a PCRE2_SIZE variable can hold. However, the program that calls pcre2_compile() can specify a smaller limit.
The maximum length (in code units) of a subject string is one less than the largest number a PCRE2_SIZE variable can hold. PCRE2_SIZE is an un- signed integer type, usually defined as size_t. Its maximum value (that is ~(PCRE2_SIZE)0) is reserved as a special indicator for zero-termi- nated strings and unset offsets.
All values in repeating quantifiers must be less than 65536.
There are two different limits that apply to branches of lookbehind as- sertions. If every branch in such an assertion matches a fixed number of characters, the maximum length of any branch is 65535 characters. If any branch matches a variable number of characters, then the maximum matching length for every branch is limited. The default limit is set at compile time, defaulting to 255, but can be changed by the calling program.
There is no limit to the number of parenthesized groups, but there can be no more than 65535 capture groups, and there is a limit to the depth of nesting of parenthesized subpatterns of all kinds. This is imposed in order to limit the amount of system stack used at compile time. The default limit can be specified when PCRE2 is built; if not, the default is set to 250. An application can change this limit by calling pcre2_set_parens_nest_limit() to set the limit in a compile context.
The maximum length of name for a named capture group is 32 code units, and the maximum number of such groups is 10000.
The maximum length of a name in a (*MARK), (*PRUNE), (*SKIP), or (*THEN) verb is 255 code units for the 8-bit library and 65535 code units for the 16-bit and 32-bit libraries.
The maximum length of a string argument to a callout is the largest number a 32-bit unsigned integer can hold.
The maximum amount of heap memory used for matching is controlled by the heap limit, which can be set in a pattern or in a match context. The default is a very large number, effectively unlimited.
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
REVISION
Last updated: August 2023 Copyright (c) 1997-2023 University of Cambridge.
PCRE2 10.43 1 August 2023 PCRE2LIMITS(3)------------------------------------------------------------------------------
PCRE2MATCHING(3) Library Functions Manual PCRE2MATCHING(3)
NAME PCRE2 - Perl-compatible regular expressions (revised API)
PCRE2 MATCHING ALGORITHMS
This document describes the two different algorithms that are available in PCRE2 for matching a compiled regular expression against a given subject string. The "standard" algorithm is the one provided by the pcre2_match() function. This works in the same as Perl's matching func- tion, and provide a Perl-compatible matching operation. The just-in- time (JIT) optimization that is described in the pcre2jit documentation is compatible with this function.
An alternative algorithm is provided by the pcre2_dfa_match() function; it operates in a different way, and is not Perl-compatible. This alter- native has advantages and disadvantages compared with the standard al- gorithm, and these are described below.
When there is only one possible way in which a given subject string can match a pattern, the two algorithms give the same answer. A difference arises, however, when there are multiple possibilities. For example, if the pattern
^<.*>
is matched against the string
<something> <something else> <something further>
there are three possible answers. The standard algorithm finds only one of them, whereas the alternative algorithm finds all three.
REGULAR EXPRESSIONS AS TREES
The set of strings that are matched by a regular expression can be rep- resented as a tree structure. An unlimited repetition in the pattern makes the tree of infinite size, but it is still a tree. Matching the pattern to a given subject string (from a given starting point) can be thought of as a search of the tree. There are two ways to search a tree: depth-first and breadth-first, and these correspond to the two matching algorithms provided by PCRE2.
THE STANDARD MATCHING ALGORITHM
In the terminology of Jeffrey Friedl's book "Mastering Regular Expres- sions", the standard algorithm is an "NFA algorithm". It conducts a depth-first search of the pattern tree. That is, it proceeds along a single path through the tree, checking that the subject matches what is required. When there is a mismatch, the algorithm tries any alterna- tives at the current point, and if they all fail, it backs up to the previous branch point in the tree, and tries the next alternative branch at that level. This often involves backing up (moving to the left) in the subject string as well. The order in which repetition branches are tried is controlled by the greedy or ungreedy nature of the quantifier.
If a leaf node is reached, a matching string has been found, and at that point the algorithm stops. Thus, if there is more than one possi- ble match, this algorithm returns the first one that it finds. Whether this is the shortest, the longest, or some intermediate length depends on the way the alternations and the greedy or ungreedy repetition quan- tifiers are specified in the pattern.
Because it ends up with a single path through the tree, it is rela- tively straightforward for this algorithm to keep track of the sub- strings that are matched by portions of the pattern in parentheses. This provides support for capturing parentheses and backreferences.
THE ALTERNATIVE MATCHING ALGORITHM
This algorithm conducts a breadth-first search of the tree. Starting from the first matching point in the subject, it scans the subject string from left to right, once, character by character, and as it does this, it remembers all the paths through the tree that represent valid matches. In Friedl's terminology, this is a kind of "DFA algorithm", though it is not implemented as a traditional finite state machine (it keeps multiple states active simultaneously).
Although the general principle of this matching algorithm is that it scans the subject string only once, without backtracking, there is one exception: when a lookaround assertion is encountered, the characters following or preceding the current point have to be independently in- spected.
The scan continues until either the end of the subject is reached, or there are no more unterminated paths. At this point, terminated paths represent the different matching possibilities (if there are none, the match has failed). Thus, if there is more than one possible match, this algorithm finds all of them, and in particular, it finds the longest. The matches are returned in the output vector in decreasing order of length. There is an option to stop the algorithm after the first match (which is necessarily the shortest) is found.
Note that the size of vector needed to contain all the results depends on the number of simultaneous matches, not on the number of parentheses in the pattern. Using pcre2_match_data_create_from_pattern() to create the match data block is therefore not advisable when doing DFA match- ing.
Note also that all the matches that are found start at the same point in the subject. If the pattern
cat(er(pillar)?)?
is matched against the string "the caterpillar catchment", the result is the three strings "caterpillar", "cater", and "cat" that start at the fifth character of the subject. The algorithm does not automati- cally move on to find matches that start at later positions.
PCRE2's "auto-possessification" optimization usually applies to charac- ter repeats at the end of a pattern (as well as internally). For exam- ple, the pattern "a\d+" is compiled as if it were "a\d++" because there is no point even considering the possibility of backtracking into the repeated digits. For DFA matching, this means that only one possible match is found. If you really do want multiple matches in such cases, either use an ungreedy repeat ("a\d+?") or set the PCRE2_NO_AUTO_POS- SESS option when compiling.
There are a number of features of PCRE2 regular expressions that are not supported or behave differently in the alternative matching func- tion. Those that are not supported cause an error if encountered.
1. Because the algorithm finds all possible matches, the greedy or un- greedy nature of repetition quantifiers is not relevant (though it may affect auto-possessification, as just described). During matching, greedy and ungreedy quantifiers are treated in exactly the same way. However, possessive quantifiers can make a difference when what follows could also match what is quantified, for example in a pattern like this:
^a++\w!
This pattern matches "aaab!" but not "aaa!", which would be matched by a non-possessive quantifier. Similarly, if an atomic group is present, it is matched as if it were a standalone pattern at the current point, and the longest match is then "locked in" for the rest of the overall pattern.
2. When dealing with multiple paths through the tree simultaneously, it is not straightforward to keep track of captured substrings for the different matching possibilities, and PCRE2's implementation of this algorithm does not attempt to do this. This means that no captured sub- strings are available.
3. Because no substrings are captured, backreferences within the pat- tern are not supported.
4. For the same reason, conditional expressions that use a backrefer- ence as the condition or test for a specific group recursion are not supported.
5. Again for the same reason, script runs are not supported.
6. Because many paths through the tree may be active, the \K escape se- quence, which resets the start of the match when encountered (but may be on some paths and not on others), is not supported.
7. Callouts are supported, but the value of the capture_top field is always 1, and the value of the capture_last field is always 0.
8. The \C escape sequence, which (in the standard algorithm) always matches a single code unit, even in a UTF mode, is not supported in these modes, because the alternative algorithm moves through the sub- ject string one character (not code unit) at a time, for all active paths through the tree.
9. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) are not supported. (*FAIL) is supported, and behaves like a failing negative assertion.
10. The PCRE2_MATCH_INVALID_UTF option for pcre2_compile() is not sup- ported by pcre2_dfa_match().
ADVANTAGES OF THE ALTERNATIVE ALGORITHM
The main advantage of the alternative algorithm is that all possible matches (at a single point in the subject) are automatically found, and in particular, the longest match is found. To find more than one match at the same point using the standard algorithm, you have to do kludgy things with callouts.
Partial matching is possible with this algorithm, though it has some limitations. The pcre2partial documentation gives details of partial matching and discusses multi-segment matching.
DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
The alternative algorithm suffers from a number of disadvantages:
1. It is substantially slower than the standard algorithm. This is partly because it has to search for all possible matches, but is also because it is less susceptible to optimization.
2. Capturing parentheses, backreferences, script runs, and matching within invalid UTF string are not supported.
3. Although atomic groups are supported, their use does not provide the performance advantage that it does for the standard algorithm.
4. JIT optimization is not supported.
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
REVISION
Last updated: 19 January 2024 Copyright (c) 1997-2024 University of Cambridge.
PCRE2 10.43 19 January 2024 PCRE2MATCHING(3)------------------------------------------------------------------------------
PCRE2PARTIAL(3) Library Functions Manual PCRE2PARTIAL(3)
NAME PCRE2 - Perl-compatible regular expressions
PARTIAL MATCHING IN PCRE2
In normal use of PCRE2, if there is a match up to the end of a subject string, but more characters are needed to match the entire pattern, PCRE2_ERROR_NOMATCH is returned, just like any other failing match. There are circumstances where it might be helpful to distinguish this "partial match" case.
One example is an application where the subject string is very long, and not all available at once. The requirement here is to be able to do the matching segment by segment, but special action is needed when a matched substring spans the boundary between two segments.
Another example is checking a user input string as it is typed, to en- sure that it conforms to a required format. Invalid characters can be immediately diagnosed and rejected, giving instant feedback.
Partial matching is a PCRE2-specific feature; it is not Perl-compati- ble. It is requested by setting one of the PCRE2_PARTIAL_HARD or PCRE2_PARTIAL_SOFT options when calling a matching function. The dif- ference between the two options is whether or not a partial match is preferred to an alternative complete match, though the details differ between the two types of matching function. If both options are set, PCRE2_PARTIAL_HARD takes precedence.
If you want to use partial matching with just-in-time optimized code, as well as setting a partial match option for the matching function, you must also call pcre2_jit_compile() with one or both of these op- tions:
PCRE2_JIT_PARTIAL_HARD PCRE2_JIT_PARTIAL_SOFT
PCRE2_JIT_COMPLETE should also be set if you are going to run non-par- tial matches on the same pattern. Separate code is compiled for each mode. If the appropriate JIT mode has not been compiled, interpretive matching code is used.
Setting a partial matching option disables two of PCRE2's standard op- timization hints. PCRE2 remembers the last literal code unit in a pat- tern, and abandons matching immediately if it is not present in the subject string. This optimization cannot be used for a subject string that might match only partially. PCRE2 also remembers a minimum length of a matching string, and does not bother to run the matching function on shorter strings. This optimization is also disabled for partial matching.
REQUIREMENTS FOR A PARTIAL MATCH
A possible partial match occurs during matching when the end of the subject string is reached successfully, but either more characters are needed to complete the match, or the addition of more characters might change what is matched.
Example 1: if the pattern is /abc/ and the subject is "ab", more char- acters are definitely needed to complete a match. In this case both hard and soft matching options yield a partial match.
Example 2: if the pattern is /ab+/ and the subject is "ab", a complete match can be found, but the addition of more characters might change what is matched. In this case, only PCRE2_PARTIAL_HARD returns a par- tial match; PCRE2_PARTIAL_SOFT returns the complete match.
On reaching the end of the subject, when PCRE2_PARTIAL_HARD is set, if the next pattern item is \z, \Z, \b, \B, or $ there is always a partial match. Otherwise, for both options, the next pattern item must be one that inspects a character, and at least one of the following must be true:
(1) At least one character has already been inspected. An inspected character need not form part of the final matched string; lookbehind assertions and the \K escape sequence provide ways of inspecting char- acters before the start of a matched string.
(2) The pattern contains one or more lookbehind assertions. This condi- tion exists in case there is a lookbehind that inspects characters be- fore the start of the match.
(3) There is a special case when the whole pattern can match an empty string. When the starting point is at the end of the subject, the empty string match is a possibility, and if PCRE2_PARTIAL_SOFT is set and neither of the above conditions is true, it is returned. However, because adding more characters might result in a non-empty match, PCRE2_PARTIAL_HARD returns a partial match, which in this case means "there is going to be a match at this point, but until some more char- acters are added, we do not know if it will be an empty string or some- thing longer".
PARTIAL MATCHING USING pcre2_match()
When a partial matching option is set, the result of calling pcre2_match() can be one of the following:
A successful match A complete match has been found, starting and ending within this sub- ject.
PCRE2_ERROR_NOMATCH No match can start anywhere in this subject.
PCRE2_ERROR_PARTIAL Adding more characters may result in a complete match that uses one or more characters from the end of this subject.
When a partial match is returned, the first two elements in the ovector point to the portion of the subject that was matched, but the values in the rest of the ovector are undefined. The appearance of \K in the pat- tern has no effect for a partial match. Consider this pattern:
/abc\K123/
If it is matched against "456abc123xyz" the result is a complete match, and the ovector defines the matched string as "123", because \K resets the "start of match" point. However, if a partial match is requested and the subject string is "456abc12", a partial match is found for the string "abc12", because all these characters are needed for a subse- quent re-match with additional characters.
If there is more than one partial match, the first one that was found provides the data that is returned. Consider this pattern:
/123\w+X|dogY/
If this is matched against the subject string "abc123dog", both alter- natives fail to match, but the end of the subject is reached during matching, so PCRE2_ERROR_PARTIAL is returned. The offsets are set to 3 and 9, identifying "123dog" as the first partial match. (In this exam- ple, there are two partial matches, because "dog" on its own partially matches the second alternative.)
How a partial match is processed by pcre2_match()
What happens when a partial match is identified depends on which of the two partial matching options is set.
If PCRE2_PARTIAL_HARD is set, PCRE2_ERROR_PARTIAL is returned as soon as a partial match is found, without continuing to search for possible complete matches. This option is "hard" because it prefers an earlier partial match over a later complete match. For this reason, the assump- tion is made that the end of the supplied subject string is not the true end of the available data, which is why \z, \Z, \b, \B, and $ al- ways give a partial match.
If PCRE2_PARTIAL_SOFT is set, the partial match is remembered, but matching continues as normal, and other alternatives in the pattern are tried. If no complete match can be found, PCRE2_ERROR_PARTIAL is re- turned instead of PCRE2_ERROR_NOMATCH. This option is "soft" because it prefers a complete match over a partial match. All the various matching items in a pattern behave as if the subject string is potentially com- plete; \z, \Z, and $ match at the end of the subject, as normal, and for \b and \B the end of the subject is treated as a non-alphanumeric.
The difference between the two partial matching options can be illus- trated by a pattern such as:
/dog(sbody)?/
This matches either "dog" or "dogsbody", greedily (that is, it prefers the longer string if possible). If it is matched against the string "dog" with PCRE2_PARTIAL_SOFT, it yields a complete match for "dog". However, if PCRE2_PARTIAL_HARD is set, the result is PCRE2_ERROR_PAR- TIAL. On the other hand, if the pattern is made ungreedy the result is different:
/dog(sbody)??/
In this case the result is always a complete match because that is found first, and matching never continues after finding a complete match. It might be easier to follow this explanation by thinking of the two patterns like this:
/dog(sbody)?/ is the same as /dogsbody|dog/ /dog(sbody)??/ is the same as /dog|dogsbody/
The second pattern will never match "dogsbody", because it will always find the shorter match first.
Example of partial matching using pcre2test
The pcre2test data modifiers partial_hard (or ph) and partial_soft (or ps) set PCRE2_PARTIAL_HARD and PCRE2_PARTIAL_SOFT, respectively, when calling pcre2_match(). Here is a run of pcre2test using a pattern that matches the whole subject in the form of a date:
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ data> 25dec3\=ph Partial match: 23dec3 data> 3ju\=ph Partial match: 3ju data> 3juj\=ph No match
This example gives the same results for both hard and soft partial matching options. Here is an example where there is a difference:
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ data> 25jun04\=ps 0: 25jun04 1: jun data> 25jun04\=ph Partial match: 25jun04
With PCRE2_PARTIAL_SOFT, the subject is matched completely. For PCRE2_PARTIAL_HARD, however, the subject is assumed not to be complete, so there is only a partial match.
MULTI-SEGMENT MATCHING WITH pcre2_match()
PCRE was not originally designed with multi-segment matching in mind. However, over time, features (including partial matching) that make multi-segment matching possible have been added. A very long string can be searched segment by segment by calling pcre2_match() repeatedly, with the aim of achieving the same results that would happen if the en- tire string was available for searching all the time. Normally, the strings that are being sought are much shorter than each individual segment, and are in the middle of very long strings, so the pattern is normally not anchored.
Special logic must be implemented to handle a matched substring that spans a segment boundary. PCRE2_PARTIAL_HARD should be used, because it returns a partial match at the end of a segment whenever there is the possibility of changing the match by adding more characters. The PCRE2_NOTBOL option should also be set for all but the first segment.
When a partial match occurs, the next segment must be added to the cur- rent subject and the match re-run, using the startoffset argument of pcre2_match() to begin at the point where the partial match started. For example:
re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/ data> ...the date is 23ja\=ph Partial match: 23ja data> ...the date is 23jan19 and on that day...\=offset=15 0: 23jan19 1: jan
Note the use of the offset modifier to start the new match where the partial match was found. In this example, the next segment was added to the one in which the partial match was found. This is the most straightforward approach, typically using a memory buffer that is twice the size of each segment. After a partial match, the first half of the buffer is discarded, the second half is moved to the start of the buffer, and a new segment is added before repeating the match as in the example above. After a no match, the entire buffer can be discarded.
If there are memory constraints, you may want to discard text that pre- cedes a partial match before adding the next segment. Unfortunately, this is not at present straightforward. In cases such as the above, where the pattern does not contain any lookbehinds, it is sufficient to retain only the partially matched substring. However, if the pattern contains a lookbehind assertion, characters that precede the start of the partial match may have been inspected during the matching process. When pcre2test displays a partial match, it indicates these characters with '<' if the allusedtext modifier is set:
re> "(?<=123)abc" data> xx123ab\=ph,allusedtext Partial match: 123ab <<<
However, the allusedtext modifier is not available for JIT matching, because JIT matching does not record the first (or last) consulted characters. For this reason, this information is not available via the API. It is therefore not possible in general to obtain the exact number of characters that must be retained in order to get the right match re- sult. If you cannot retain the entire segment, you must find some heuristic way of choosing.
If you know the approximate length of the matching substrings, you can use that to decide how much text to retain. The only lookbehind infor- mation that is currently available via the API is the length of the longest individual lookbehind in a pattern, but this can be misleading if there are nested lookbehinds. The value returned by calling pcre2_pattern_info() with the PCRE2_INFO_MAXLOOKBEHIND option is the maximum number of characters (not code units) that any individual look- behind moves back when it is processed. A pattern such as "(?<=(?<!b)a)" has a maximum lookbehind value of one, but inspects two characters before its starting point.
In a non-UTF or a 32-bit case, moving back is just a subtraction, but in UTF-8 or UTF-16 you have to count characters while moving back through the code units.
PARTIAL MATCHING USING pcre2_dfa_match()
The DFA function moves along the subject string character by character, without backtracking, searching for all possible matches simultane- ously. If the end of the subject is reached before the end of the pat- tern, there is the possibility of a partial match.
When PCRE2_PARTIAL_SOFT is set, PCRE2_ERROR_PARTIAL is returned only if there have been no complete matches. Otherwise, the complete matches are returned. If PCRE2_PARTIAL_HARD is set, a partial match takes precedence over any complete matches. The portion of the string that was matched when the longest partial match was found is set as the first matching string.
Because the DFA function always searches for all possible matches, and there is no difference between greedy and ungreedy repetition, its be- haviour is different from the pcre2_match(). Consider the string "dog" matched against this ungreedy pattern:
/dog(sbody)??/
Whereas the standard function stops as soon as it finds the complete match for "dog", the DFA function also finds the partial match for "dogsbody", and so returns that when PCRE2_PARTIAL_HARD is set.
MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()
When a partial match has been found using the DFA matching function, it is possible to continue the match by providing additional subject data and calling the function again with the same compiled regular expres- sion, this time setting the PCRE2_DFA_RESTART option. You must pass the same working space as before, because this is where details of the pre- vious partial match are stored. You can set the PCRE2_PARTIAL_SOFT or PCRE2_PARTIAL_HARD options with PCRE2_DFA_RESTART to continue partial matching over multiple segments. Here is an example using pcre2test:
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ data> 23ja\=dfa,ps Partial match: 23ja data> n05\=dfa,dfa_restart 0: n05
The first call has "23ja" as the subject, and requests partial match- ing; the second call has "n05" as the subject for the continued (restarted) match. Notice that when the match is complete, only the last part is shown; PCRE2 does not retain the previously partially- matched string. It is up to the calling program to do that if it needs to. This means that, for an unanchored pattern, if a continued match fails, it is not possible to try again at a new starting point. All this facility is capable of doing is continuing with the previous match attempt. For example, consider this pattern:
1234|3789
If the first part of the subject is "ABC123", a partial match of the first alternative is found at offset 3. There is no partial match for the second alternative, because such a match does not start at the same point in the subject string. Attempting to continue with the string "7890" does not yield a match because only those alternatives that match at one point in the subject are remembered. Depending on the ap- plication, this may or may not be what you want.
If you do want to allow for starting again at the next character, one way of doing it is to retain some or all of the segment and try a new complete match, as described for pcre2_match() above. Another possibil- ity is to work with two buffers. If a partial match at offset n in the first buffer is followed by "no match" when PCRE2_DFA_RESTART is used on the second buffer, you can then try a new match starting at offset n+1 in the first buffer.
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
REVISION
Last updated: 04 September 2019 Copyright (c) 1997-2019 University of Cambridge.
PCRE2 10.34 04 September 2019 PCRE2PARTIAL(3)------------------------------------------------------------------------------
PCRE2PATTERN(3) Library Functions Manual PCRE2PATTERN(3)
NAME PCRE2 - Perl-compatible regular expressions (revised API)
PCRE2 REGULAR EXPRESSION DETAILS
The syntax and semantics of the regular expressions that are supported by PCRE2 are described in detail below. There is a quick-reference syn- tax summary in the pcre2syntax page. PCRE2 tries to match Perl syntax and semantics as closely as it can. PCRE2 also supports some alterna- tive regular expression syntax (which does not conflict with the Perl syntax) in order to provide some compatibility with regular expressions in Python, .NET, and Oniguruma.
Perl's regular expressions are described in its own documentation, and regular expressions in general are covered in a number of books, some of which have copious examples. Jeffrey Friedl's "Mastering Regular Ex- pressions", published by O'Reilly, covers regular expressions in great detail. This description of PCRE2's regular expressions is intended as reference material.
This document discusses the regular expression patterns that are sup- ported by PCRE2 when its main matching function, pcre2_match(), is used. PCRE2 also has an alternative matching function, pcre2_dfa_match(), which matches using a different algorithm that is not Perl-compatible. Some of the features discussed below are not available when DFA matching is used. The advantages and disadvantages of the alternative function, and how it differs from the normal func- tion, are discussed in the pcre2matching page.
SPECIAL START-OF-PATTERN ITEMS
A number of options that can be passed to pcre2_compile() can also be set by special items at the start of a pattern. These are not Perl-com- patible, but are provided to make these options accessible to pattern writers who are not able to change the program that processes the pat- tern. Any number of these items may appear, but they must all be to- gether right at the start of the pattern string, and the letters must be in upper case.
UTF support
In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either as single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32 can be specified for the 32-bit library, in which case it constrains the character values to valid Unicode code points. To process UTF strings, PCRE2 must be built to include Unicode support (which is the default). When using UTF strings you must either call the compiling function with one or both of the PCRE2_UTF or PCRE2_MATCH_INVALID_UTF options, or the pattern must start with the special sequence (*UTF), which is equivalent to setting the relevant PCRE2_UTF. How setting a UTF mode affects pattern matching is mentioned in several places below. There is also a summary of features in the pcre2unicode page.
Some applications that allow their users to supply patterns may wish to restrict them to non-UTF data for security reasons. If the PCRE2_NEVER_UTF option is passed to pcre2_compile(), (*UTF) is not al- lowed, and its appearance in a pattern causes an error.
Unicode property support
Another special sequence that may appear at the start of a pattern is (*UCP). This has the same effect as setting the PCRE2_UCP option: it causes sequences such as \d and \w to use Unicode properties to deter- mine character types, instead of recognizing only characters with codes less than 256 via a lookup table. If also causes upper/lower casing op- erations to use Unicode properties for characters with code points greater than 127, even when UTF is not set. These behaviours can be changed within the pattern; see the section entitled "Internal Option Setting" below.
Some applications that allow their users to supply patterns may wish to restrict them for security reasons. If the PCRE2_NEVER_UCP option is passed to pcre2_compile(), (*UCP) is not allowed, and its appearance in a pattern causes an error.
Locking out empty string matching
Starting a pattern with (*NOTEMPTY) or (*NOTEMPTY_ATSTART) has the same effect as passing the PCRE2_NOTEMPTY or PCRE2_NOTEMPTY_ATSTART option to whichever matching function is subsequently called to match the pat- tern. These options lock out the matching of empty strings, either en- tirely, or only at the start of the subject.
Disabling auto-possessification
If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting the PCRE2_NO_AUTO_POSSESS option. This stops PCRE2 from making quantifiers possessive when what follows cannot match the repeated item. For example, by default a+b is treated as a++b. For more details, see the pcre2api documentation.
Disabling start-up optimizations
If a pattern starts with (*NO_START_OPT), it has the same effect as setting the PCRE2_NO_START_OPTIMIZE option. This disables several opti- mizations for quickly reaching "no match" results. For more details, see the pcre2api documentation.
Disabling automatic anchoring
If a pattern starts with (*NO_DOTSTAR_ANCHOR), it has the same effect as setting the PCRE2_NO_DOTSTAR_ANCHOR option. This disables optimiza- tions that apply to patterns whose top-level branches all start with .* (match any number of arbitrary characters). For more details, see the pcre2api documentation.
Disabling JIT compilation
If a pattern that starts with (*NO_JIT) is successfully compiled, an attempt by the application to apply the JIT optimization by calling pcre2_jit_compile() is ignored.
Setting match resource limits
The pcre2_match() function contains a counter that is incremented every time it goes round its main loop. The caller of pcre2_match() can set a limit on this counter, which therefore limits the amount of computing resource used for a match. The maximum depth of nested backtracking can also be limited; this indirectly restricts the amount of heap memory that is used, but there is also an explicit memory limit that can be set.
These facilities are provided to catch runaway matches that are pro- voked by patterns with huge matching trees. A common example is a pat- tern with nested unlimited repeats applied to a long string that does not match. When one of these limits is reached, pcre2_match() gives an error return. The limits can also be set by items at the start of the pattern of the form
(*LIMIT_HEAP=d) (*LIMIT_MATCH=d) (*LIMIT_DEPTH=d)
where d is any number of decimal digits. However, the value of the set- ting must be less than the value set (or defaulted) by the caller of pcre2_match() for it to have any effect. In other words, the pattern writer can lower the limits set by the programmer, but not raise them. If there is more than one setting of one of these limits, the lower value is used. The heap limit is specified in kibibytes (units of 1024 bytes).
Prior to release 10.30, LIMIT_DEPTH was called LIMIT_RECURSION. This name is still recognized for backwards compatibility.
The heap limit applies only when the pcre2_match() or pcre2_dfa_match() interpreters are used for matching. It does not apply to JIT. The match limit is used (but in a different way) when JIT is being used, or when pcre2_dfa_match() is called, to limit computing resource usage by those matching functions. The depth limit is ignored by JIT but is relevant for DFA matching, which uses function recursion for recursions within the pattern and for lookaround assertions and atomic groups. In this case, the depth limit controls the depth of such recursion.
Newline conventions
PCRE2 supports six different conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (line- feed) character, the two-character sequence CRLF, any of the three pre- ceding, any Unicode newline sequence, or the NUL character (binary zero). The pcre2api page has further discussion about newlines, and shows how to set the newline convention when calling pcre2_compile().
It is also possible to specify a newline convention by starting a pat- tern string with one of the following sequences:
(*CR) carriage return (*LF) linefeed (*CRLF) carriage return, followed by linefeed (*ANYCRLF) any of the three above (*ANY) all Unicode newline sequences (*NUL) the NUL character (binary zero)
These override the default and the options given to the compiling func- tion. For example, on a Unix system where LF is the default newline se- quence, the pattern
(*CR)a.b
changes the convention to CR. That pattern matches "a\nb" because LF is no longer a newline. If more than one of these settings is present, the last one is used.
The newline convention affects where the circumflex and dollar asser- tions are true. It also affects the interpretation of the dot metachar- acter when PCRE2_DOTALL is not set, and the behaviour of \N when not followed by an opening brace. However, it does not affect what the \R escape sequence matches. By default, this is any Unicode newline se- quence, for Perl compatibility. However, this can be changed; see the next section and the description of \R in the section entitled "Newline sequences" below. A change of \R setting can be combined with a change of newline convention.
Specifying what \R matches
It is possible to restrict \R to match only CR, LF, or CRLF (instead of the complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF at compile time. This effect can also be achieved by starting a pattern with (*BSR_ANYCRLF). For completeness, (*BSR_UNI- CODE) is also recognized, corresponding to PCRE2_BSR_UNICODE.
EBCDIC CHARACTER CODES
PCRE2 can be compiled to run in an environment that uses EBCDIC as its character code instead of ASCII or Unicode (typically a mainframe sys- tem). In the sections below, character code values are ASCII or Uni- code; in an EBCDIC environment these characters may have different code values, and there are no code points greater than 255.
CHARACTERS AND METACHARACTERS
A regular expression is a pattern that is matched against a subject string from left to right. Most characters stand for themselves in a pattern, and match the corresponding characters in the subject. As a trivial example, the pattern
The quick brown fox
matches a portion of a subject string that is identical to itself. When caseless matching is specified (the PCRE2_CASELESS option or (?i) within the pattern), letters are matched independently of case. Note that there are two ASCII characters, K and S, that, in addition to their lower case ASCII equivalents, are case-equivalent with Unicode U+212A (Kelvin sign) and U+017F (long S) respectively when either PCRE2_UTF or PCRE2_UCP is set, unless the PCRE2_EXTRA_CASELESS_RESTRICT option is in force (either passed to pcre2_compile() or set by (?r) within the pattern).
The power of regular expressions comes from the ability to include wild cards, character classes, alternatives, and repetitions in the pattern. These are encoded in the pattern by the use of metacharacters, which do not stand for themselves but instead are interpreted in some special way.
There are two different sets of metacharacters: those that are recog- nized anywhere in the pattern except within square brackets, and those that are recognized within square brackets. Outside square brackets, the metacharacters are as follows:
\ general escape character with several uses ^ assert start of string (or line, in multiline mode) $ assert end of string (or line, in multiline mode) . match any character except newline (by default) [ start character class definition | start of alternative branch ( start group or control verb ) end group or control verb * 0 or more quantifier + 1 or more quantifier; also "possessive quantifier" ? 0 or 1 quantifier; also quantifier minimizer { potential start of min/max quantifier
Brace characters { and } are also used to enclose data for construc- tions such as \g{2} or \k{name}. In almost all uses of braces, space and/or horizontal tab characters that follow { or precede } are allowed and are ignored. In the case of quantifiers, they may also appear be- fore or after the comma. The exception to this is \u{...} which is an ECMAScript compatibility feature that is recognized only when the PCRE2_EXTRA_ALT_BSUX option is set. ECMAScript does not ignore such white space; it causes the item to be interpreted as literal.
Part of a pattern that is in square brackets is called a "character class". In a character class the only metacharacters are:
\ general escape character ^ negate the class, but only if the first character - indicates character range [ POSIX character class (if followed by POSIX syntax) ] terminates the character class
If a pattern is compiled with the PCRE2_EXTENDED option, most white space in the pattern, other than in a character class, within a \Q...\E sequence, or between a # outside a character class and the next new- line, inclusive, are ignored. An escaping backslash can be used to in- clude a white space or a # character as part of the pattern. If the PCRE2_EXTENDED_MORE option is set, the same applies, but in addition unescaped space and horizontal tab characters are ignored inside a character class. Note: only these two characters are ignored, not the full set of pattern white space characters that are ignored outside a character class. Option settings can be changed within a pattern; see the section entitled "Internal Option Setting" below.
The following sections describe the use of each of the metacharacters.
BACKSLASH
The backslash character has several uses. Firstly, if it is followed by a character that is not a digit or a letter, it takes away any special meaning that character may have. This use of backslash as an escape character applies both inside and outside character classes.
For example, if you want to match a * character, you must write \* in the pattern. This escaping action applies whether or not the following character would otherwise be interpreted as a metacharacter, so it is always safe to precede a non-alphanumeric with backslash to specify that it stands for itself. In particular, if you want to match a back- slash, you write \\.
Only ASCII digits and letters have any special meaning after a back- slash. All other characters (in particular, those whose code points are greater than 127) are treated as literals.
If you want to treat all characters in a sequence as literals, you can do so by putting them between \Q and \E. Note that this includes white space even when the PCRE2_EXTENDED option is set so that most other white space is ignored. The behaviour is different from Perl in that $ and @ are handled as literals in \Q...\E sequences in PCRE2, whereas in Perl, $ and @ cause variable interpolation. Also, Perl does "double- quotish backslash interpolation" on any backslashes between \Q and \E which, its documentation says, "may lead to confusing results". PCRE2 treats a backslash between \Q and \E just like any other character. Note the following examples:
Pattern PCRE2 matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the contents of $xyz \Qabc\$xyz\E abc\$xyz abc\$xyz \Qabc\E\$\Qxyz\E abc$xyz abc$xyz \QA\B\E A\B A\B \Q\\E \ \\E
The \Q...\E sequence is recognized both inside and outside character classes. An isolated \E that is not preceded by \Q is ignored. If \Q is not followed by \E later in the pattern, the literal interpretation continues to the end of the pattern (that is, \E is assumed at the end). If the isolated \Q is inside a character class, this causes an error, because the character class is then not terminated by a closing square bracket.
Non-printing characters
A second use of backslash provides a way of encoding non-printing char- acters in patterns in a visible manner. There is no restriction on the appearance of non-printing characters in a pattern, but when a pattern is being prepared by text editing, it is often easier to use one of the following escape sequences instead of the binary character it repre- sents. In an ASCII or Unicode environment, these escapes are as fol- lows:
\a alarm, that is, the BEL character (hex 07) \cx "control-x", where x is a non-control ASCII character \e escape (hex 1B) \f form feed (hex 0C) \n linefeed (hex 0A) \r carriage return (hex 0D) (but see below) \t tab (hex 09) \0dd character with octal code 0dd \ddd character with octal code ddd, or backreference \o{ddd..} character with octal code ddd.. \xhh character with hex code hh \x{hhh..} character with hex code hhh.. \N{U+hhh..} character with Unicode hex code point hhh..
By default, after \x that is not followed by {, from zero to two hexa- decimal digits are read (letters can be in upper or lower case). Any number of hexadecimal digits may appear between \x{ and }. If a charac- ter other than a hexadecimal digit appears between \x{ and }, or if there is no terminating }, an error occurs.
Characters whose code points are less than 256 can be defined by either of the two syntaxes for \x or by an octal sequence. There is no differ- ence in the way they are handled. For example, \xdc is exactly the same as \x{dc} or \334. However, using the braced versions does make such sequences easier to read.
Support is available for some ECMAScript (aka JavaScript) escape se- quences via two compile-time options. If PCRE2_ALT_BSUX is set, the se- quence \x followed by { is not recognized. Only if \x is followed by two hexadecimal digits is it recognized as a character escape. Other- wise it is interpreted as a literal "x" character. In this mode, sup- port for code points greater than 256 is provided by \u, which must be followed by four hexadecimal digits; otherwise it is interpreted as a literal "u" character.
PCRE2_EXTRA_ALT_BSUX has the same effect as PCRE2_ALT_BSUX and, in ad- dition, \u{hhh..} is recognized as the character specified by hexadeci- mal code point. There may be any number of hexadecimal digits, but un- like other places that also use curly brackets, spaces are not allowed and would result in the string being interpreted as a literal. This syntax is from ECMAScript 6.
The \N{U+hhh..} escape sequence is recognized only when PCRE2 is oper- ating in UTF mode. Perl also uses \N{name} to specify characters by Unicode name; PCRE2 does not support this. Note that when \N is not followed by an opening brace (curly bracket) it has an entirely differ- ent meaning, matching any character that is not a newline.
There are some legacy applications where the escape sequence \r is ex- pected to match a newline. If the PCRE2_EXTRA_ESCAPED_CR_IS_LF option is set, \r in a pattern is converted to \n so that it matches a LF (linefeed) instead of a CR (carriage return) character.
An error occurs if \c is not followed by a character whose ASCII code point is in the range 32 to 126. The precise effect of \cx is as fol- lows: if x is a lower case letter, it is converted to upper case. Then bit 6 of the character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the code unit following \c has a code point less than 32 or greater than 126, a compile-time error occurs.
When PCRE2 is compiled in EBCDIC mode, \N{U+hhh..} is not supported. \a, \e, \f, \n, \r, and \t generate the appropriate EBCDIC code values. The \c escape is processed as specified for Perl in the perlebcdic doc- ument. The only characters that are allowed after \c are A-Z, a-z, or one of @, [, \, ], ^, _, or ?. Any other character provokes a compile- time error. The sequence \c@ encodes character code 0; after \c the letters (in either case) encode characters 1-26 (hex 01 to hex 1A); [, \, ], ^, and _ encode characters 27-31 (hex 1B to hex 1F), and \c? be- comes either 255 (hex FF) or 95 (hex 5F).
Thus, apart from \c?, these escapes generate the same character code values as they do in an ASCII environment, though the meanings of the values mostly differ. For example, \cG always generates code value 7, which is BEL in ASCII but DEL in EBCDIC.
The sequence \c? generates DEL (127, hex 7F) in an ASCII environment, but because 127 is not a control character in EBCDIC, Perl makes it generate the APC character. Unfortunately, there are several variants of EBCDIC. In most of them the APC character has the value 255 (hex FF), but in the one Perl calls POSIX-BC its value is 95 (hex 5F). If certain other characters have POSIX-BC values, PCRE2 makes \c? generate 95; otherwise it generates 255.
After \0 up to two further octal digits are read. If there are fewer than two digits, just those that are present are used. Thus the se- quence \0\x\015 specifies two binary zeros followed by a CR character (code value 13). Make sure you supply two digits after the initial zero if the pattern character that follows is itself an octal digit.
The escape \o must be followed by a sequence of octal digits, enclosed in braces. An error occurs if this is not the case. This escape is a recent addition to Perl; it provides way of specifying character code points as octal numbers greater than 0777, and it also allows octal numbers and backreferences to be unambiguously specified.
For greater clarity and unambiguity, it is best to avoid following \ by a digit greater than zero. Instead, use \o{...} or \x{...} to specify numerical character code points, and \g{...} to specify backreferences. The following paragraphs describe the old, ambiguous syntax.
The handling of a backslash followed by a digit other than 0 is compli- cated, and Perl has changed over time, causing PCRE2 also to change.
Outside a character class, PCRE2 reads the digit and any following dig- its as a decimal number. If the number is less than 10, begins with the digit 8 or 9, or if there are at least that many previous capture groups in the expression, the entire sequence is taken as a backrefer- ence. A description of how this works is given later, following the discussion of parenthesized groups. Otherwise, up to three octal dig- its are read to form a character code.
Inside a character class, PCRE2 handles \8 and \9 as the literal char- acters "8" and "9", and otherwise reads up to three octal digits fol- lowing the backslash, using them to generate a data character. Any sub- sequent digits stand for themselves. For example, outside a character class:
\040 is another way of writing an ASCII space \40 is the same, provided there are fewer than 40 previous capture groups \7 is always a backreference \11 might be a backreference, or another way of writing a tab \011 is always a tab \0113 is a tab followed by the character "3" \113 might be a backreference, otherwise the character with octal code 113 \377 might be a backreference, otherwise the value 255 (decimal) \81 is always a backreference
Note that octal values of 100 or greater that are specified using this syntax must not be introduced by a leading zero, because no more than three octal digits are ever read.
Constraints on character values
Characters that are specified using octal or hexadecimal numbers are limited to certain values, as follows:
8-bit non-UTF mode no greater than 0xff 16-bit non-UTF mode no greater than 0xffff 32-bit non-UTF mode no greater than 0xffffffff All UTF modes no greater than 0x10ffff and a valid code point
Invalid Unicode code points are all those in the range 0xd800 to 0xdfff (the so-called "surrogate" code points). The check for these can be disabled by the caller of pcre2_compile() by setting the option PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES. However, this is possible only in UTF-8 and UTF-32 modes, because these values are not representable in UTF-16.
Escape sequences in character classes
All the sequences that define a single character value can be used both inside and outside character classes. In addition, inside a character class, \b is interpreted as the backspace character (hex 08).
When not followed by an opening brace, \N is not allowed in a character class. \B, \R, and \X are not special inside a character class. Like other unrecognized alphabetic escape sequences, they cause an error. Outside a character class, these sequences have different meanings.
Unsupported escape sequences
In Perl, the sequences \F, \l, \L, \u, and \U are recognized by its string handler and used to modify the case of following characters. By default, PCRE2 does not support these escape sequences in patterns. However, if either of the PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX op- tions is set, \U matches a "U" character, and \u can be used to define a character by code point, as described above.
Absolute and relative backreferences
The sequence \g followed by a signed or unsigned number, optionally en- closed in braces, is an absolute or relative backreference. A named backreference can be coded as \g{name}. Backreferences are discussed later, following the discussion of parenthesized groups.
Absolute and relative subroutine calls
For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either in angle brackets or single quotes, is an alternative syntax for referencing a capture group as a subroutine. Details are discussed later. Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former is a backref- erence; the latter is a subroutine call.
Generic character types
Another use of backslash is for specifying generic character types:
\d any decimal digit \D any character that is not a decimal digit \h any horizontal white space character \H any character that is not a horizontal white space character \N any character that is not a newline \s any white space character \S any character that is not a white space character \v any vertical white space character \V any character that is not a vertical white space character \w any "word" character \W any "non-word" character
The \N escape sequence has the same meaning as the "." metacharacter when PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not change the meaning of \N. Note that when \N is followed by an opening brace it has a different meaning. See the section entitled "Non-printing charac- ters" above for details. Perl also uses \N{name} to specify characters by Unicode name; PCRE2 does not support this.
Each pair of lower and upper case escape sequences partitions the com- plete set of characters into two disjoint sets. Any given character matches one, and only one, of each pair. The sequences can appear both inside and outside character classes. They each match one character of the appropriate type. If the current matching point is at the end of the subject string, all of them fail, because there is no character to match.
The default \s characters are HT (9), LF (10), VT (11), FF (12), CR (13), and space (32), which are defined as white space in the "C" lo- cale. This list may vary if locale-specific matching is taking place. For example, in some locales the "non-breaking space" character (\xA0) is recognized as white space, and in others the VT character is not.
A "word" character is an underscore or any character that is a letter or digit. By default, the definition of letters and digits is con- trolled by PCRE2's low-valued character tables, and may vary if locale- specific matching is taking place (see "Locale support" in the pcre2api page). For example, in a French locale such as "fr_FR" in Unix-like systems, or "french" in Windows, some character codes greater than 127 are used for accented letters, and these are then matched by \w. The use of locales with Unicode is discouraged.
By default, characters whose code points are greater than 127 never match \d, \s, or \w, and always match \D, \S, and \W, although this may be different for characters in the range 128-255 when locale-specific matching is happening. These escape sequences retain their original meanings from before Unicode support was available, mainly for effi- ciency reasons. If the PCRE2_UCP option is set, the behaviour is changed so that Unicode properties are used to determine character types, as follows:
\d any character that matches \p{Nd} (decimal digit) \s any character that matches \p{Z} or \h or \v \w any character that matches \p{L}, \p{N}, \p{Mn}, or \p{Pc}
The addition of \p{Mn} (non-spacing mark) and the replacement of an ex- plicit test for underscore with a test for \p{Pc} (connector punctua- tion) happened in PCRE2 release 10.43. This brings PCRE2 into line with Perl.
The upper case escapes match the inverse sets of characters. Note that \d matches only decimal digits, whereas \w matches any Unicode digit, as well as other character categories. Note also that PCRE2_UCP affects \b, and \B because they are defined in terms of \w and \W. Matching these sequences is noticeably slower when PCRE2_UCP is set.
The effect of PCRE2_UCP on any one of these escape sequences can be negated by the options PCRE2_EXTRA_ASCII_BSD, PCRE2_EXTRA_ASCII_BSS, and PCRE2_EXTRA_ASCII_BSW, respectively. These options can be set and reset within a pattern by means of an internal option setting (see be- low).
The sequences \h, \H, \v, and \V, in contrast to the other sequences, which match only ASCII characters by default, always match a specific list of code points, whether or not PCRE2_UCP is set. The horizontal space characters are:
U+0009 Horizontal tab (HT) U+0020 Space U+00A0 Non-break space U+1680 Ogham space mark U+180E Mongolian vowel separator U+2000 En quad U+2001 Em quad U+2002 En space U+2003 Em space U+2004 Three-per-em space U+2005 Four-per-em space U+2006 Six-per-em space U+2007 Figure space U+2008 Punctuation space U+2009 Thin space U+200A Hair space U+202F Narrow no-break space U+205F Medium mathematical space U+3000 Ideographic space
The vertical space characters are:
U+000A Linefeed (LF) U+000B Vertical tab (VT) U+000C Form feed (FF) U+000D Carriage return (CR) U+0085 Next line (NEL) U+2028 Line separator U+2029 Paragraph separator
In 8-bit, non-UTF-8 mode, only the characters with code points less than 256 are relevant.
Newline sequences
Outside a character class, by default, the escape sequence \R matches any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the following:
(?>\r\n|\n|\x0b|\f|\r|\x85)
This is an example of an "atomic group", details of which are given be- low. This particular group matches either the two-character sequence CR followed by LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (car- riage return, U+000D), or NEL (next line, U+0085). Because this is an atomic group, the two-character sequence is treated as a single unit that cannot be split.
In other modes, two additional characters whose code points are greater than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa- rator, U+2029). Unicode support is not needed for these characters to be recognized.
It is possible to restrict \R to match only CR, LF, or CRLF (instead of the complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF at compile time. (BSR is an abbreviation for "back- slash R".) This can be made the default when PCRE2 is built; if this is the case, the other behaviour can be requested via the PCRE2_BSR_UNI- CODE option. It is also possible to specify these settings by starting a pattern string with one of the following sequences:
(*BSR_ANYCRLF) CR, LF, or CRLF only (*BSR_UNICODE) any Unicode newline sequence
These override the default and the options given to the compiling func- tion. Note that these special settings, which are not Perl-compatible, are recognized only at the very start of a pattern, and that they must be in upper case. If more than one of them is present, the last one is used. They can be combined with a change of newline convention; for ex- ample, a pattern can start with:
(*ANY)(*BSR_ANYCRLF)
They can also be combined with the (*UTF) or (*UCP) special sequences. Inside a character class, \R is treated as an unrecognized escape se- quence, and causes an error.
Unicode character properties
When PCRE2 is built with Unicode support (the default), three addi- tional escape sequences that match characters with specific properties are available. They can be used in any mode, though in 8-bit and 16-bit non-UTF modes these sequences are of course limited to testing charac- ters whose code points are less than U+0100 and U+10000, respectively. In 32-bit non-UTF mode, code points greater than 0x10ffff (the Unicode limit) may be encountered. These are all treated as being in the Un- known script and with an unassigned type.
Matching characters by Unicode property is not fast, because PCRE2 has to do a multistage table lookup in order to find a character's prop- erty. That is why the traditional escape sequences such as \d and \w do not use Unicode properties in PCRE2 by default, though you can make them do so by setting the PCRE2_UCP option or by starting the pattern with (*UCP).
The extra escape sequences that provide property support are:
\p{xx} a character with the xx property \P{xx} a character without the xx property \X a Unicode extended grapheme cluster
The property names represented by xx above are not case-sensitive, and in accordance with Unicode's "loose matching" rules, spaces, hyphens, and underscores are ignored. There is support for Unicode script names, Unicode general category properties, "Any", which matches any character (including newline), Bidi_Class, a number of binary (yes/no) proper- ties, and some special PCRE2 properties (described below). Certain other Perl properties such as "InMusicalSymbols" are not supported by PCRE2. Note that \P{Any} does not match any characters, so always causes a match failure.
Script properties for \p and \P
There are three different syntax forms for matching a script. Each Uni- code character has a basic script and, optionally, a list of other scripts ("Script Extensions") with which it is commonly used. Using the Adlam script as an example, \p{sc:Adlam} matches characters whose basic script is Adlam, whereas \p{scx:Adlam} matches, in addition, characters that have Adlam in their extensions list. The full names "script" and "script extensions" for the property types are recognized, and a equals sign is an alternative to the colon. If a script name is given without a property type, for example, \p{Adlam}, it is treated as \p{scx:Ad- lam}. Perl changed to this interpretation at release 5.26 and PCRE2 changed at release 10.40.
Unassigned characters (and in non-UTF 32-bit mode, characters with code points greater than 0x10FFFF) are assigned the "Unknown" script. Others that are not part of an identified script are lumped together as "Com- mon". The current list of recognized script names and their 4-character abbreviations can be obtained by running this command:
pcre2test -LS
The general category property for \p and \P
Each character has exactly one Unicode general category property, spec- ified by a two-letter abbreviation. For compatibility with Perl, nega- tion can be specified by including a circumflex between the opening brace and the property name. For example, \p{^Lu} is the same as \P{Lu}.
If only one letter is specified with \p or \P, it includes all the gen- eral category properties that start with that letter. In this case, in the absence of negation, the curly brackets in the escape sequence are optional; these two examples have the same effect:
\p{L} \pL
The following general category property codes are supported:
C Other Cc Control Cf Format Cn Unassigned Co Private use Cs Surrogate
L Letter Ll Lower case letter Lm Modifier letter Lo Other letter Lt Title case letter Lu Upper case letter
M Mark Mc Spacing mark Me Enclosing mark Mn Non-spacing mark
N Number Nd Decimal number Nl Letter number No Other number
P Punctuation Pc Connector punctuation Pd Dash punctuation Pe Close punctuation Pf Final punctuation Pi Initial punctuation Po Other punctuation Ps Open punctuation
S Symbol Sc Currency symbol Sk Modifier symbol Sm Mathematical symbol So Other symbol
Z Separator Zl Line separator Zp Paragraph separator Zs Space separator
The special property LC, which has the synonym L&, is also supported: it matches a character that has the Lu, Ll, or Lt property, in other words, a letter that is not classified as a modifier or "other".
The Cs (Surrogate) property applies only to characters whose code points are in the range U+D800 to U+DFFF. These characters are no dif- ferent to any other character when PCRE2 is not in UTF mode (using the 16-bit or 32-bit library). However, they are not valid in Unicode strings and so cannot be tested by PCRE2 in UTF mode, unless UTF valid- ity checking has been turned off (see the discussion of PCRE2_NO_UTF_CHECK in the pcre2api page).
The long synonyms for property names that Perl supports (such as \p{Letter}) are not supported by PCRE2, nor is it permitted to prefix any of these properties with "Is".
No character that is in the Unicode table has the Cn (unassigned) prop- erty. Instead, this property is assumed for any code point that is not in the Unicode table.
Specifying caseless matching does not affect these escape sequences. For example, \p{Lu} always matches only upper case letters. This is different from the behaviour of current versions of Perl.
Binary (yes/no) properties for \p and \P
Unicode defines a number of binary properties, that is, properties whose only values are true or false. You can obtain a list of those that are recognized by \p and \P, along with their abbreviations, by running this command:
pcre2test -LP
The Bidi_Class property for \p and \P
\p{Bidi_Class:<class>} matches a character with the given class \p{BC:<class>} matches a character with the given class
The recognized classes are:
AL Arabic letter AN Arabic number B paragraph separator BN boundary neutral CS common separator EN European number ES European separator ET European terminator FSI first strong isolate L left-to-right LRE left-to-right embedding LRI left-to-right isolate LRO left-to-right override NSM non-spacing mark ON other neutral PDF pop directional format PDI pop directional isolate R right-to-left RLE right-to-left embedding RLI right-to-left isolate RLO right-to-left override S segment separator WS which space
An equals sign may be used instead of a colon. The class names are case-insensitive; only the short names listed above are recognized.
Extended grapheme clusters
The \X escape matches any number of Unicode characters that form an "extended grapheme cluster", and treats the sequence as an atomic group (see below). Unicode supports various kinds of composite character by giving each character a grapheme breaking property, and having rules that use these properties to define the boundaries of extended grapheme clusters. The rules are defined in Unicode Standard Annex 29, "Unicode Text Segmentation". Unicode 11.0.0 abandoned the use of some previous properties that had been used for emojis. Instead it introduced vari- ous emoji-specific properties. PCRE2 uses only the Extended Picto- graphic property.
\X always matches at least one character. Then it decides whether to add additional characters according to the following rules for ending a cluster:
1. End at the end of the subject string.
2. Do not end between CR and LF; otherwise end after any control char- acter.
3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters are of five types: L, V, T, LV, and LVT. An L character may be followed by an L, V, LV, or LVT character; an LV or V character may be followed by a V or T character; an LVT or T character may be fol- lowed only by a T character.
4. Do not end before extending characters or spacing marks or the zero- width joiner (ZWJ) character. Characters with the "mark" property al- ways have the "extend" grapheme breaking property.
5. Do not end after prepend characters.
6. Do not end within emoji modifier sequences or emoji ZWJ (zero-width joiner) sequences. An emoji ZWJ sequence consists of a character with the Extended_Pictographic property, optionally followed by one or more characters with the Extend property, followed by the ZWJ character, followed by another Extended_Pictographic character.
7. Do not break within emoji flag sequences. That is, do not break be- tween regional indicator (RI) characters if there are an odd number of RI characters before the break point.
8. Otherwise, end the cluster.
PCRE2's additional properties
As well as the standard Unicode properties described above, PCRE2 sup- ports four more that make it possible to convert traditional escape se- quences such as \w and \s to use Unicode properties. PCRE2 uses these non-standard, non-Perl properties internally when PCRE2_UCP is set. However, they may also be used explicitly. These properties are:
Xan Any alphanumeric character Xps Any POSIX space character Xsp Any Perl space character Xwd Any Perl "word" character
Xan matches characters that have either the L (letter) or the N (num- ber) property. Xps matches the characters tab, linefeed, vertical tab, form feed, or carriage return, and any other character that has the Z (separator) property. Xsp is the same as Xps; in PCRE1 it used to ex- clude vertical tab, for Perl compatibility, but Perl changed. Xwd matches the same characters as Xan, plus those that match Mn (non-spac- ing mark) or Pc (connector punctuation, which includes underscore).
There is another non-standard property, Xuc, which matches any charac- ter that can be represented by a Universal Character Name in C++ and other programming languages. These are the characters $, @, ` (grave accent), and all characters with Unicode code points greater than or equal to U+00A0, except for the surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH where H is a hexadecimal digit. Note that the Xuc property does not match these sequences but the char- acters that they represent.)
Resetting the match start
In normal use, the escape sequence \K causes any previously matched characters not to be included in the final matched sequence that is re- turned. For example, the pattern:
foo\Kbar
matches "foobar", but reports that it has matched "bar". \K does not interact with anchoring in any way. The pattern:
^foo\Kbar
matches only when the subject begins with "foobar" (in single line mode), though it again reports the matched string as "bar". This fea- ture is similar to a lookbehind assertion (described below), but the part of the pattern that precedes \K is not constrained to match a lim- ited number of characters, as is required for a lookbehind assertion. The use of \K does not interfere with the setting of captured sub- strings. For example, when the pattern
(foo)\Kbar
matches "foobar", the first substring is still set to "foo".
From version 5.32.0 Perl forbids the use of \K in lookaround asser- tions. From release 10.38 PCRE2 also forbids this by default. However, the PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK option can be used when calling pcre2_compile() to re-enable the previous behaviour. When this option is set, \K is acted upon when it occurs inside positive assertions, but is ignored in negative assertions. Note that when a pattern such as (?=ab\K) matches, the reported start of the match can be greater than the end of the match. Using \K in a lookbehind assertion at the start of a pattern can also lead to odd effects. For example, consider this pattern:
(?<=\Kfoo)bar
If the subject is "foobar", a call to pcre2_match() with a starting offset of 3 succeeds and reports the matching string as "foobar", that is, the start of the reported match is earlier than where the match started.
Simple assertions
The final use of backslash is for certain simple assertions. An asser- tion specifies a condition that has to be met at a particular point in a match, without consuming any characters from the subject string. The use of groups for more complicated assertions is described below. The backslashed assertions are:
\b matches at a word boundary \B matches when not at a word boundary \A matches at the start of the subject \Z matches at the end of the subject also matches before a newline at the end of the subject \z matches only at the end of the subject \G matches at the first matching position in the subject
Inside a character class, \b has a different meaning; it matches the backspace character. If any other of these assertions appears in a character class, an "invalid escape sequence" error is generated.
A word boundary is a position in the subject string where the current character and the previous character do not both match \w or \W (i.e. one matches \w and the other matches \W), or the start or end of the string if the first or last character matches \w, respectively. When PCRE2 is built with Unicode support, the meanings of \w and \W can be changed by setting the PCRE2_UCP option. When this is done, it also af- fects \b and \B. Neither PCRE2 nor Perl has a separate "start of word" or "end of word" metasequence. However, whatever follows \b normally determines which it is. For example, the fragment \ba matches "a" at the start of a word.
The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described in the next section) in that they only ever match at the very start and end of the subject string, whatever options are set. Thus, they are independent of multiline mode. These three asser- tions are not affected by the PCRE2_NOTBOL or PCRE2_NOTEOL options, which affect only the behaviour of the circumflex and dollar metachar- acters. However, if the startoffset argument of pcre2_match() is non- zero, indicating that matching is to start at a point other than the beginning of the subject, \A can never match. The difference between \Z and \z is that \Z matches before a newline at the end of the string as well as at the very end, whereas \z matches only at the end.
The \G assertion is true only when the current matching position is at the start point of the matching process, as specified by the startoff- set argument of pcre2_match(). It differs from \A when the value of startoffset is non-zero. By calling pcre2_match() multiple times with appropriate arguments, you can mimic Perl's /g option, and it is in this kind of implementation where \G can be useful.
Note, however, that PCRE2's implementation of \G, being true at the starting character of the matching process, is subtly different from Perl's, which defines it as true at the end of the previous match. In Perl, these can be different when the previously matched string was empty. Because PCRE2 does just one match at a time, it cannot reproduce this behaviour.
If all the alternatives of a pattern begin with \G, the expression is anchored to the starting match position, and the "anchored" flag is set in the compiled regular expression.
CIRCUMFLEX AND DOLLAR
The circumflex and dollar metacharacters are zero-width assertions. That is, they test for a particular condition being true without con- suming any characters from the subject string. These two metacharacters are concerned with matching the starts and ends of lines. If the new- line convention is set so that only the two-character sequence CRLF is recognized as a newline, isolated CR and LF characters are treated as ordinary data characters, and are not recognized as newlines.
Outside a character class, in the default matching mode, the circumflex character is an assertion that is true only if the current matching point is at the start of the subject string. If the startoffset argu- ment of pcre2_match() is non-zero, or if PCRE2_NOTBOL is set, circum- flex can never match if the PCRE2_MULTILINE option is unset. Inside a character class, circumflex has an entirely different meaning (see be- low).
Circumflex need not be the first character of the pattern if a number of alternatives are involved, but it should be the first thing in each alternative in which it appears if the pattern is ever to match that branch. If all possible alternatives start with a circumflex, that is, if the pattern is constrained to match only at the start of the sub- ject, it is said to be an "anchored" pattern. (There are also other constructs that can cause a pattern to be anchored.)
The dollar character is an assertion that is true only if the current matching point is at the end of the subject string, or immediately be- fore a newline at the end of the string (by default), unless PCRE2_NO- TEOL is set. Note, however, that it does not actually match the new- line. Dollar need not be the last character of the pattern if a number of alternatives are involved, but it should be the last item in any branch in which it appears. Dollar has no special meaning in a charac- ter class.
The meaning of dollar can be changed so that it matches only at the very end of the string, by setting the PCRE2_DOLLAR_ENDONLY option at compile time. This does not affect the \Z assertion.
The meanings of the circumflex and dollar metacharacters are changed if the PCRE2_MULTILINE option is set. When this is the case, a dollar character matches before any newlines in the string, as well as at the very end, and a circumflex matches immediately after internal newlines as well as at the start of the subject string. It does not match after a newline that ends the string, for compatibility with Perl. However, this can be changed by setting the PCRE2_ALT_CIRCUMFLEX option.
For example, the pattern /^abc$/ matches the subject string "def\nabc" (where \n represents a newline) in multiline mode, but not otherwise. Consequently, patterns that are anchored in single line mode because all branches start with ^ are not anchored in multiline mode, and a match for circumflex is possible when the startoffset argument of pcre2_match() is non-zero. The PCRE2_DOLLAR_ENDONLY option is ignored if PCRE2_MULTILINE is set.
When the newline convention (see "Newline conventions" below) recog- nizes the two-character sequence CRLF as a newline, this is preferred, even if the single characters CR and LF are also recognized as new- lines. For example, if the newline convention is "any", a multiline mode circumflex matches before "xyz" in the string "abc\r\nxyz" rather than after CR, even though CR on its own is a valid newline. (It also matches at the very start of the string, of course.)
Note that the sequences \A, \Z, and \z can be used to match the start and end of the subject in both modes, and if all branches of a pattern start with \A it is always anchored, whether or not PCRE2_MULTILINE is set.
FULL STOP (PERIOD, DOT) AND \N
Outside a character class, a dot in the pattern matches any one charac- ter in the subject string except (by default) a character that signi- fies the end of a line. One or more characters may be specified as line terminators (see "Newline conventions" above).
Dot never matches a single line-ending character. When the two-charac- ter sequence CRLF is the only line ending, dot does not match CR if it is immediately followed by LF, but otherwise it matches all characters (including isolated CRs and LFs). When ANYCRLF is selected for line endings, no occurrences of CR of LF match dot. When all Unicode line endings are being recognized, dot does not match CR or LF or any of the other line ending characters.
The behaviour of dot with regard to newlines can be changed. If the PCRE2_DOTALL option is set, a dot matches any one character, without exception. If the two-character sequence CRLF is present in the sub- ject string, it takes two dots to match it.
The handling of dot is entirely independent of the handling of circum- flex and dollar, the only relationship being that they both involve newlines. Dot has no special meaning in a character class.
The escape sequence \N when not followed by an opening brace behaves like a dot, except that it is not affected by the PCRE2_DOTALL option. In other words, it matches any character except one that signifies the end of a line.
When \N is followed by an opening brace it has a different meaning. See the section entitled "Non-printing characters" above for details. Perl also uses \N{name} to specify characters by Unicode name; PCRE2 does not support this.
MATCHING A SINGLE CODE UNIT
Outside a character class, the escape sequence \C matches any one code unit, whether or not a UTF mode is set. In the 8-bit library, one code unit is one byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is a 32-bit unit. Unlike a dot, \C always matches line-ending characters. The feature is provided in Perl in order to match individual bytes in UTF-8 mode, but it is unclear how it can use- fully be used.
Because \C breaks up characters into individual code units, matching one unit with \C in UTF-8 or UTF-16 mode means that the rest of the string may start with a malformed UTF character. This has undefined re- sults, because PCRE2 assumes that it is matching character by character in a valid UTF string (by default it checks the subject string's valid- ity at the start of processing unless the PCRE2_NO_UTF_CHECK or PCRE2_MATCH_INVALID_UTF option is used).
An application can lock out the use of \C by setting the PCRE2_NEVER_BACKSLASH_C option when compiling a pattern. It is also possible to build PCRE2 with the use of \C permanently disabled.
PCRE2 does not allow \C to appear in lookbehind assertions (described below) in UTF-8 or UTF-16 modes, because this would make it impossible to calculate the length of the lookbehind. Neither the alternative matching function pcre2_dfa_match() nor the JIT optimizer support \C in these UTF modes. The former gives a match-time error; the latter fails to optimize and so the match is always run using the interpreter.
In the 32-bit library, however, \C is always supported (when not ex- plicitly locked out) because it always matches a single code unit, whether or not UTF-32 is specified.
In general, the \C escape sequence is best avoided. However, one way of using it that avoids the problem of malformed UTF-8 or UTF-16 charac- ters is to use a lookahead to check the length of the next character, as in this pattern, which could be used with a UTF-8 string (ignore white space and line breaks):
(?| (?=[\x00-\x7f])(\C) | (?=[\x80-\x{7ff}])(\C)(\C) | (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
In this example, a group that starts with (?| resets the capturing parentheses numbers in each alternative (see "Duplicate Group Numbers" below). The assertions at the start of each branch check the next UTF-8 character for values whose encoding uses 1, 2, 3, or 4 bytes, respec- tively. The character's individual bytes are then captured by the ap- propriate number of \C groups.
SQUARE BRACKETS AND CHARACTER CLASSES
An opening square bracket introduces a character class, terminated by a closing square bracket. A closing square bracket on its own is not spe- cial by default. If a closing square bracket is required as a member of the class, it should be the first data character in the class (after an initial circumflex, if present) or escaped with a backslash. This means that, by default, an empty class cannot be defined. However, if the PCRE2_ALLOW_EMPTY_CLASS option is set, a closing square bracket at the start does end the (empty) class.
A character class matches a single character in the subject. A matched character must be in the set of characters defined by the class, unless the first character in the class definition is a circumflex, in which case the subject character must not be in the set defined by the class. If a circumflex is actually required as a member of the class, ensure it is not the first character, or escape it with a backslash.
For example, the character class [aeiou] matches any lower case vowel, while [^aeiou] matches any character that is not a lower case vowel. Note that a circumflex is just a convenient notation for specifying the characters that are in the class by enumerating those that are not. A class that starts with a circumflex is not an assertion; it still con- sumes a character from the subject string, and therefore it fails if the current pointer is at the end of the string.
Characters in a class may be specified by their code points using \o, \x, or \N{U+hh..} in the usual way. When caseless matching is set, any letters in a class represent both their upper case and lower case ver- sions, so for example, a caseless [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a caseful version would. Note that there are two ASCII characters, K and S, that, in ad- dition to their lower case ASCII equivalents, are case-equivalent with Unicode U+212A (Kelvin sign) and U+017F (long S) respectively when ei- ther PCRE2_UTF or PCRE2_UCP is set.
Characters that might indicate line breaks are never treated in any special way when matching character classes, whatever line-ending se- quence is in use, and whatever setting of the PCRE2_DOTALL and PCRE2_MULTILINE options is used. A class such as [^a] always matches one of these characters.
The generic character type escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, \w, and \W may appear in a character class, and add the characters that they match to the class. For example, [\dABCDEF] matches any hexadecimal digit. In UTF modes, the PCRE2_UCP option af- fects the meanings of \d, \s, \w and their upper case partners, just as it does when they appear outside a character class, as described in the section entitled "Generic character types" above. The escape sequence \b has a different meaning inside a character class; it matches the backspace character. The sequences \B, \R, and \X are not special in- side a character class. Like any other unrecognized escape sequences, they cause an error. The same is true for \N when not followed by an opening brace.
The minus (hyphen) character can be used to specify a range of charac- ters in a character class. For example, [d-m] matches any letter be- tween d and m, inclusive. If a minus character is required in a class, it must be escaped with a backslash or appear in a position where it cannot be interpreted as indicating a range, typically as the first or last character in the class, or immediately after a range. For example, [b-d-z] matches letters in the range b to d, a hyphen character, or z.
Perl treats a hyphen as a literal if it appears before or after a POSIX class (see below) or before or after a character type escape such as \d or \H. However, unless the hyphen is the last character in the class, Perl outputs a warning in its warning mode, as this is most likely a user error. As PCRE2 has no facility for warning, an error is given in these cases.
It is not possible to have the literal character "]" as the end charac- ter of a range. A pattern such as [W-]46] is interpreted as a class of two characters ("W" and "-") followed by a literal string "46]", so it would match "W46]" or "-46]". However, if the "]" is escaped with a backslash it is interpreted as the end of range, so [W-\]46] is inter- preted as a class containing a range followed by two other characters. The octal or hexadecimal representation of "]" can also be used to end a range.
Ranges normally include all code points between the start and end char- acters, inclusive. They can also be used for code points specified nu- merically, for example [\000-\037]. Ranges can include any characters that are valid for the current mode. In any UTF mode, the so-called "surrogate" characters (those whose code points lie between 0xd800 and 0xdfff inclusive) may not be specified explicitly by default (the PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES option disables this check). How- ever, ranges such as [\x{d7ff}-\x{e000}], which include the surrogates, are always permitted.
There is a special case in EBCDIC environments for ranges whose end points are both specified as literal letters in the same case. For com- patibility with Perl, EBCDIC code points within the range that are not letters are omitted. For example, [h-k] matches only four characters, even though the codes for h and k are 0x88 and 0x92, a range of 11 code points. However, if the range is specified numerically, for example, [\x88-\x92] or [h-\x92], all code points are included.
If a range that includes letters is used when caseless matching is set, it matches the letters in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character tables for a French locale are in use, [\xc8-\xcb] matches accented E characters in both cases.
A circumflex can conveniently be used with the upper case character types to specify a more restricted set of characters than the matching lower case type. For example, the class [^\W_] matches any letter or digit, but not underscore, whereas [\w] includes underscore. A positive character class should be read as "something OR something OR ..." and a negative class as "NOT something AND NOT something AND NOT ...".
The only metacharacters that are recognized in character classes are backslash, hyphen (only where it can be interpreted as specifying a range), circumflex (only at the start), opening square bracket (only when it can be interpreted as introducing a POSIX class name, or for a special compatibility feature - see the next two sections), and the terminating closing square bracket. However, escaping other non-al- phanumeric characters does no harm.
POSIX CHARACTER CLASSES
Perl supports the POSIX notation for character classes. This uses names enclosed by [: and :] within the enclosing square brackets. PCRE2 also supports this notation. For example,
[01[:alpha:]%]
matches "0", "1", any alphabetic character, or "%". The supported class names are:
alnum letters and digits alpha letters ascii character codes 0 - 127 blank space or tab only cntrl control characters digit decimal digits (same as \d) graph printing characters, excluding space lower lower case letters print printing characters, including space punct printing characters, excluding letters and digits and space space white space (the same as \s from PCRE2 8.34) upper upper case letters word "word" characters (same as \w) xdigit hexadecimal digits
The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and space (32). If locale-specific matching is taking place, the list of space characters may be different; there may be fewer or more of them. "Space" and \s match the same set of characters, as do "word" and \w.
The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 5.8. Another Perl extension is negation, which is indicated by a ^ character after the colon. For example,
[12[:^digit:]]
matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not supported, and an error is given if they are encountered.
By default, characters with values greater than 127 do not match any of the POSIX character classes, although this may be different for charac- ters in the range 128-255 when locale-specific matching is happening. However, in UCP mode, unless certain options are set (see below), some of the classes are changed so that Unicode character properties are used. This is achieved by replacing POSIX classes with other sequences, as follows:
[:alnum:] becomes \p{Xan} [:alpha:] becomes \p{L} [:blank:] becomes \h [:cntrl:] becomes \p{Cc} [:digit:] becomes \p{Nd} [:lower:] becomes \p{Ll} [:space:] becomes \p{Xps} [:upper:] becomes \p{Lu} [:word:] becomes \p{Xwd}
Negated versions, such as [:^alpha:] use \P instead of \p. Four other POSIX classes are handled specially in UCP mode:
[:graph:] This matches characters that have glyphs that mark the page when printed. In Unicode property terms, it matches all char- acters with the L, M, N, P, S, or Cf properties, except for:
U+061C Arabic Letter Mark U+180E Mongolian Vowel Separator U+2066 - U+2069 Various "isolate"s
[:print:] This matches the same characters as [:graph:] plus space characters that are not controls, that is, characters with the Zs property.
[:punct:] This matches all characters that have the Unicode P (punctua- tion) property, plus those characters with code points less than 256 that have the S (Symbol) property.
[:xdigit:] In addition to the ASCII hexadecimal digits, this also matches the "fullwidth" versions of those characters, whose Unicode code points start at U+FF10. This is a change that was made in PCRE release 10.43 for Perl compatibility.
The other POSIX classes are unchanged by PCRE2_UCP, and match only characters with code points less than 256.
There are two options that can be used to restrict the POSIX classes to ASCII characters when PCRE2_UCP is set. The option PCRE2_EX- TRA_ASCII_DIGIT affects just [:digit:] and [:xdigit:]. Within a pat- tern, this can be set and unset by (?aT) and (?-aT). The PCRE2_EX- TRA_ASCII_POSIX option disables UCP processing for all POSIX classes, including [:digit:] and [:xdigit:]. Within a pattern, (?aP) and (?-aP) set and unset both these options for consistency.
COMPATIBILITY FEATURE FOR WORD BOUNDARIES
In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of word". PCRE2 treats these items as follows:
[[:<:]] is converted to \b(?=\w) [[:>:]] is converted to \b(?<=\w)
Only these exact character sequences are recognized. A sequence such as [a[:<:]b] provokes error for an unrecognized POSIX class name. This support is not compatible with Perl. It is provided to help migrations from other environments, and is best not used in any new patterns. Note that \b matches at the start and the end of a word (see "Simple asser- tions" above), and in a Perl-style pattern the preceding or following character normally shows which is wanted, without the need for the as- sertions that are used above in order to give exactly the POSIX behav- iour. Note also that the PCRE2_UCP option changes the meaning of \w (and therefore \b) by default, so it also affects these POSIX se- quences.
VERTICAL BAR
Vertical bar characters are used to separate alternative patterns. For example, the pattern
gilbert|sullivan
matches either "gilbert" or "sullivan". Any number of alternatives may appear, and an empty alternative is permitted (matching the empty string). The matching process tries each alternative in turn, from left to right, and the first one that succeeds is used. If the alternatives are within a group (defined below), "succeeds" means matching the rest of the main pattern as well as the alternative in the group.
INTERNAL OPTION SETTING
The settings of several options can be changed within a pattern by a sequence of letters enclosed between "(?" and ")". The following are Perl-compatible, and are described in detail in the pcre2api documenta- tion. The option letters are:
i for PCRE2_CASELESS m for PCRE2_MULTILINE n for PCRE2_NO_AUTO_CAPTURE s for PCRE2_DOTALL x for PCRE2_EXTENDED xx for PCRE2_EXTENDED_MORE
For example, (?im) sets caseless, multiline matching. It is also possi- ble to unset these options by preceding the relevant letters with a hy- phen, for example (?-im). The two "extended" options are not indepen- dent; unsetting either one cancels the effects of both of them.
A combined setting and unsetting such as (?im-sx), which sets PCRE2_CASELESS and PCRE2_MULTILINE while unsetting PCRE2_DOTALL and PCRE2_EXTENDED, is also permitted. Only one hyphen may appear in the options string. If a letter appears both before and after the hyphen, the option is unset. An empty options setting "(?)" is allowed. Need- less to say, it has no effect.
If the first character following (? is a circumflex, it causes all of the above options to be unset. Letters may follow the circumflex to cause some options to be re-instated, but a hyphen may not appear.
Some PCRE2-specific options can be changed by the same mechanism using these pairs or individual letters:
aD for PCRE2_EXTRA_ASCII_BSD aS for PCRE2_EXTRA_ASCII_BSS aW for PCRE2_EXTRA_ASCII_BSW aP for PCRE2_EXTRA_ASCII_POSIX and PCRE2_EXTRA_ASCII_DIGIT aT for PCRE2_EXTRA_ASCII_DIGIT r for PCRE2_EXTRA_CASELESS_RESTRICT J for PCRE2_DUPNAMES U for PCRE2_UNGREEDY
However, except for 'r', these are not unset by (?^), which is equiva- lent to (?-imnrsx). If 'a' is not followed by any of the upper case letters shown above, it sets (or unsets) all the ASCII options.
PCRE2_EXTRA_ASCII_DIGIT has no additional effect when PCRE2_EX- TRA_ASCII_POSIX is set, but including it in (?aP) means that (?-aP) suppresses all ASCII restrictions for POSIX classes.
When one of these option changes occurs at top level (that is, not in- side group parentheses), the change applies until a subsequent change, or the end of the pattern. An option change within a group (see below for a description of groups) affects only that part of the group that follows it. At the end of the group these options are reset to the state they were before the group. For example,
(a(?i)b)c
matches abc and aBc and no other strings (assuming PCRE2_CASELESS is not set externally). Any changes made in one alternative do carry on into subsequent branches within the same group. For example,
(a(?i)b|c)
matches "ab", "aB", "c", and "C", even though when matching "C" the first branch is abandoned before the option setting. This is because the effects of option settings happen at compile time. There would be some very weird behaviour otherwise.
As a convenient shorthand, if any option settings are required at the start of a non-capturing group (see the next section), the option let- ters may appear between the "?" and the ":". Thus the two patterns
(?i:saturday|sunday) (?:(?i)saturday|sunday)
match exactly the same set of strings.
Note: There are other PCRE2-specific options, applying to the whole pattern, which can be set by the application when the compiling func- tion is called. In addition, the pattern can contain special leading sequences such as (*CRLF) to override what the application has set or what has been defaulted. Details are given in the section entitled "Newline sequences" above. There are also the (*UTF) and (*UCP) leading sequences that can be used to set UTF and Unicode property modes; they are equivalent to setting the PCRE2_UTF and PCRE2_UCP options, respec- tively. However, the application can set the PCRE2_NEVER_UTF or PCRE2_NEVER_UCP options, which lock out the use of the (*UTF) and (*UCP) sequences.
GROUPS
Groups are delimited by parentheses (round brackets), which can be nested. Turning part of a pattern into a group does two things:
1. It localizes a set of alternatives. For example, the pattern
cat(aract|erpillar|)
matches "cataract", "caterpillar", or "cat". Without the parentheses, it would match "cataract", "erpillar" or an empty string.
2. It creates a "capture group". This means that, when the whole pat- tern matches, the portion of the subject string that matched the group is passed back to the caller, separately from the portion that matched the whole pattern. (This applies only to the traditional matching function; the DFA matching function does not support capturing.)
Opening parentheses are counted from left to right (starting from 1) to obtain numbers for capture groups. For example, if the string "the red king" is matched against the pattern
the ((red|white) (king|queen))
the captured substrings are "red king", "red", and "king", and are num- bered 1, 2, and 3, respectively.
The fact that plain parentheses fulfil two functions is not always helpful. There are often times when grouping is required without cap- turing. If an opening parenthesis is followed by a question mark and a colon, the group does not do any capturing, and is not counted when computing the number of any subsequent capture groups. For example, if the string "the white queen" is matched against the pattern
the ((?:red|white) (king|queen))
the captured substrings are "white queen" and "queen", and are numbered 1 and 2. The maximum number of capture groups is 65535.
As a convenient shorthand, if any option settings are required at the start of a non-capturing group, the option letters may appear between the "?" and the ":". Thus the two patterns
(?i:saturday|sunday) (?:(?i)saturday|sunday)
match exactly the same set of strings. Because alternative branches are tried from left to right, and options are not reset until the end of the group is reached, an option setting in one branch does affect sub- sequent branches, so the above patterns match "SUNDAY" as well as "Sat- urday".
DUPLICATE GROUP NUMBERS
Perl 5.10 introduced a feature whereby each alternative in a group uses the same numbers for its capturing parentheses. Such a group starts with (?| and is itself a non-capturing group. For example, consider this pattern:
(?|(Sat)ur|(Sun))day
Because the two alternatives are inside a (?| group, both sets of cap- turing parentheses are numbered one. Thus, when the pattern matches, you can look at captured substring number one, whichever alternative matched. This construct is useful when you want to capture part, but not all, of one of a number of alternatives. Inside a (?| group, paren- theses are numbered as usual, but the number is reset at the start of each branch. The numbers of any capturing parentheses that follow the whole group start after the highest number used in any branch. The fol- lowing example is taken from the Perl documentation. The numbers under- neath show in which buffer the captured content will be stored.
# before ---------------branch-reset----------- after / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x # 1 2 2 3 2 3 4
A backreference to a capture group uses the most recent value that is set for the group. The following pattern matches "abcabc" or "defdef":
/(?|(abc)|(def))\1/
In contrast, a subroutine call to a capture group always refers to the first one in the pattern with the given number. The following pattern matches "abcabc" or "defabc":
/(?|(abc)|(def))(?1)/
A relative reference such as (?-1) is no different: it is just a conve- nient way of computing an absolute group number.
If a condition test for a group's having matched refers to a non-unique number, the test is true if any group with that number has matched.
An alternative approach to using this "branch reset" feature is to use duplicate named groups, as described in the next section.
NAMED CAPTURE GROUPS
Identifying capture groups by number is simple, but it can be very hard to keep track of the numbers in complicated patterns. Furthermore, if an expression is modified, the numbers may change. To help with this difficulty, PCRE2 supports the naming of capture groups. This feature was not added to Perl until release 5.10. Python had the feature ear- lier, and PCRE1 introduced it at release 4.0, using the Python syntax. PCRE2 supports both the Perl and the Python syntax.
In PCRE2, a capture group can be named in one of three ways: (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python. Names may be up to 128 code units long. When PCRE2_UTF is not set, they may contain only ASCII alphanumeric characters and underscores, but must start with a non-digit. When PCRE2_UTF is set, the syntax of group names is extended to allow any Unicode letter or Unicode decimal digit. In other words, group names must match one of these patterns:
^[_A-Za-z][_A-Za-z0-9]*\z when PCRE2_UTF is not set ^[_\p{L}][_\p{L}\p{Nd}]*\z when PCRE2_UTF is set
References to capture groups from other parts of the pattern, such as backreferences, recursion, and conditions, can all be made by name as well as by number.
Named capture groups are allocated numbers as well as names, exactly as if the names were not present. In both PCRE2 and Perl, capture groups are primarily identified by numbers; any names are just aliases for these numbers. The PCRE2 API provides function calls for extracting the complete name-to-number translation table from a compiled pattern, as well as convenience functions for extracting captured substrings by name.
Warning: When more than one capture group has the same number, as de- scribed in the previous section, a name given to one of them applies to all of them. Perl allows identically numbered groups to have different names. Consider this pattern, where there are two capture groups, both numbered 1:
(?|(?<AA>aa)|(?<BB>bb))
Perl allows this, with both names AA and BB as aliases of group 1. Thus, after a successful match, both names yield the same value (either "aa" or "bb").
In an attempt to reduce confusion, PCRE2 does not allow the same group number to be associated with more than one name. The example above pro- vokes a compile-time error. However, there is still scope for confu- sion. Consider this pattern:
(?|(?<AA>aa)|(bb))
Although the second group number 1 is not explicitly named, the name AA is still an alias for any group 1. Whether the pattern matches "aa" or "bb", a reference by name to group AA yields the matched string.
By default, a name must be unique within a pattern, except that dupli- cate names are permitted for groups with the same number, for example:
(?|(?<AA>aa)|(?<AA>bb))
The duplicate name constraint can be disabled by setting the PCRE2_DUP- NAMES option at compile time, or by the use of (?J) within the pattern, as described in the section entitled "Internal Option Setting" above.
Duplicate names can be useful for patterns where only one instance of the named capture group can match. Suppose you want to match the name of a weekday, either as a 3-letter abbreviation or as the full name, and in both cases you want to extract the abbreviation. This pattern (ignoring the line breaks) does the job:
(?J) (?<DN>Mon|Fri|Sun)(?:day)?| (?<DN>Tue)(?:sday)?| (?<DN>Wed)(?:nesday)?| (?<DN>Thu)(?:rsday)?| (?<DN>Sat)(?:urday)?
There are five capture groups, but only one is ever set after a match. The convenience functions for extracting the data by name returns the substring for the first (and in this example, the only) group of that name that matched. This saves searching to find which numbered group it was. (An alternative way of solving this problem is to use a "branch reset" group, as described in the previous section.)
If you make a backreference to a non-unique named group from elsewhere in the pattern, the groups to which the name refers are checked in the order in which they appear in the overall pattern. The first one that is set is used for the reference. For example, this pattern matches both "foofoo" and "barbar" but not "foobar" or "barfoo":
(?J)(?:(?<n>foo)|(?<n>bar))\k<n>
If you make a subroutine call to a non-unique named group, the one that corresponds to the first occurrence of the name is used. In the absence of duplicate numbers this is the one with the lowest number.
If you use a named reference in a condition test (see the section about conditions below), either to check whether a capture group has matched, or to check for recursion, all groups with the same name are tested. If the condition is true for any one of them, the overall condition is true. This is the same behaviour as testing by number. For further de- tails of the interfaces for handling named capture groups, see the pcre2api documentation.
REPETITION
Repetition is specified by quantifiers, which may follow any one of these items:
a literal data character the dot metacharacter the \C escape sequence the \R escape sequence the \X escape sequence any escape sequence that matches a single character a character class a backreference a parenthesized group (including lookaround assertions) a subroutine call (recursive or otherwise)
If a quantifier does not follow a repeatable item, an error occurs. The general repetition quantifier specifies a minimum and maximum number of permitted matches by giving two numbers in curly brackets (braces), separated by a comma. The numbers must be less than 65536, and the first must be less than or equal to the second. For example,
z{2,4}
matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special character. If the second number is omitted, but the comma is present, there is no upper limit; if the second number and the comma are both omitted, the quantifier specifies an exact number of required matches. Thus
[aeiou]{3,}
matches at least 3 successive vowels, but may match many more, whereas
\d{8}
matches exactly 8 digits. If the first number is omitted, the lower limit is taken as zero; in this case the upper limit must be present.
X{,4} is interpreted as X{0,4}
This is a change in behaviour that happened in Perl 5.34.0 and PCRE2 10.43. In earlier versions such a sequence was not interpreted as a quantifier. Other regular expression engines may behave either way.
If the characters that follow an opening brace do not match the syntax of a quantifier, the brace is taken as a literal character. In particu- lar, this means that {,} is a literal string of three characters.
Note that not every opening brace is potentially the start of a quanti- fier because braces are used in other items such as \N{U+345} or \k{name}.
In UTF modes, quantifiers apply to characters rather than to individual code units. Thus, for example, \x{100}{2} matches two characters, each of which is represented by a two-byte sequence in a UTF-8 string. Simi- larly, \X{3} matches three Unicode extended grapheme clusters, each of which may be several code units long (and they may be of different lengths).
The quantifier {0} is permitted, causing the expression to behave as if the previous item and the quantifier were not present. This may be use- ful for capture groups that are referenced as subroutines from else- where in the pattern (but see also the section entitled "Defining cap- ture groups for use by reference only" below). Except for parenthesized groups, items that have a {0} quantifier are omitted from the compiled pattern.
For convenience, the three most common quantifiers have single-charac- ter abbreviations:
* is equivalent to {0,} + is equivalent to {1,} ? is equivalent to {0,1}
It is possible to construct infinite loops by following a group that can match no characters with a quantifier that has no upper limit, for example:
(a?)*
Earlier versions of Perl and PCRE1 used to give an error at compile time for such patterns. However, because there are cases where this can be useful, such patterns are now accepted, but whenever an iteration of such a group matches no characters, matching moves on to the next item in the pattern instead of repeatedly matching an empty string. This does not prevent backtracking into any of the iterations if a subse- quent item fails to match.
By default, quantifiers are "greedy", that is, they match as much as possible (up to the maximum number of permitted repetitions), without causing the rest of the pattern to fail. The classic example of where this gives problems is in trying to match comments in C programs. These appear between /* and */ and within the comment, individual * and / characters may appear. An attempt to match C comments by applying the pattern
/\*.*\*/
to the string
/* first comment */ not comment /* second comment */
fails, because it matches the entire string owing to the greediness of the .* item. However, if a quantifier is followed by a question mark, it ceases to be greedy, and instead matches the minimum number of times possible, so the pattern
/\*.*?\*/
does the right thing with C comments. The meaning of the various quan- tifiers is not otherwise changed, just the preferred number of matches. Do not confuse this use of question mark with its use as a quantifier in its own right. Because it has two uses, it can sometimes appear doubled, as in
\d??\d
which matches one digit by preference, but can match two if that is the only way the rest of the pattern matches.
If the PCRE2_UNGREEDY option is set (an option that is not available in Perl), the quantifiers are not greedy by default, but individual ones can be made greedy by following them with a question mark. In other words, it inverts the default behaviour.
When a parenthesized group is quantified with a minimum repeat count that is greater than 1 or with a limited maximum, more memory is re- quired for the compiled pattern, in proportion to the size of the mini- mum or maximum.
If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option (equivalent to Perl's /s) is set, thus allowing the dot to match new- lines, the pattern is implicitly anchored, because whatever follows will be tried against every character position in the subject string, so there is no point in retrying the overall match at any position af- ter the first. PCRE2 normally treats such a pattern as though it were preceded by \A.
In cases where it is known that the subject string contains no new- lines, it is worth setting PCRE2_DOTALL in order to obtain this opti- mization, or alternatively, using ^ to indicate anchoring explicitly.
However, there are some cases where the optimization cannot be used. When .* is inside capturing parentheses that are the subject of a backreference elsewhere in the pattern, a match at the start may fail where a later one succeeds. Consider, for example:
(.*)abc\1
If the subject is "xyz123abc123" the match point is the fourth charac- ter. For this reason, such a pattern is not implicitly anchored.
Another case where implicit anchoring is not applied is when the lead- ing .* is inside an atomic group. Once again, a match at the start may fail where a later one succeeds. Consider this pattern:
(?>.*?a)b
It matches "ab" in the subject "aab". The use of the backtracking con- trol verbs (*PRUNE) and (*SKIP) also disable this optimization, and there is an option, PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly.
When a capture group is repeated, the value captured is the substring that matched the final iteration. For example, after
(tweedle[dume]{3}\s*)+
has matched "tweedledum tweedledee" the value of the captured substring is "tweedledee". However, if there are nested capture groups, the cor- responding captured values may have been set in previous iterations. For example, after
(a|(b))+
matches "aba" the value of the second captured substring is "b".
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") repetition, failure of what follows normally causes the repeated item to be re-evaluated to see if a different number of repeats allows the rest of the pattern to match. Sometimes it is useful to prevent this, either to change the nature of the match, or to cause it fail earlier than it otherwise might, when the author of the pattern knows there is no point in carrying on.
Consider, for example, the pattern \d+foo when applied to the subject line
123456bar
After matching all 6 digits and then failing to match "foo", the normal action of the matcher is to try again with only 5 digits matching the \d+ item, and then with 4, and so on, before ultimately failing. "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides the means for specifying that once a group has matched, it is not to be re-evaluated in this way.
If we use atomic grouping for the previous example, the matcher gives up immediately on failing to match "foo" the first time. The notation is a kind of special parenthesis, starting with (?> as in this example:
(?>\d+)foo
Perl 5.28 introduced an experimental alphabetic form starting with (* which may be easier to remember:
(*atomic:\d+)foo
This kind of parenthesized group "locks up" the part of the pattern it contains once it has matched, and a failure further into the pattern is prevented from backtracking into it. Backtracking past it to previous items, however, works as normal.
An alternative description is that a group of this type matches exactly the string of characters that an identical standalone pattern would match, if anchored at the current point in the subject string.
Atomic groups are not capture groups. Simple cases such as the above example can be thought of as a maximizing repeat that must swallow everything it can. So, while both \d+ and \d+? are prepared to adjust the number of digits they match in order to make the rest of the pat- tern match, (?>\d+) can only match an entire sequence of digits.
Atomic groups in general can of course contain arbitrarily complicated expressions, and can be nested. However, when the contents of an atomic group is just a single repeated item, as in the example above, a sim- pler notation, called a "possessive quantifier" can be used. This con- sists of an additional + character following a quantifier. Using this notation, the previous example can be rewritten as
\d++foo
Note that a possessive quantifier can be used with an entire group, for example:
(abc|xyz){2,3}+
Possessive quantifiers are always greedy; the setting of the PCRE2_UN- GREEDY option is ignored. They are a convenient notation for the sim- pler forms of atomic group. However, there is no difference in the meaning of a possessive quantifier and the equivalent atomic group, though there may be a performance difference; possessive quantifiers should be slightly faster.
The possessive quantifier syntax is an extension to the Perl 5.8 syn- tax. Jeffrey Friedl originated the idea (and the name) in the first edition of his book. Mike McCloskey liked it, so implemented it when he built Sun's Java package, and PCRE1 copied it from there. It found its way into Perl at release 5.10.
PCRE2 has an optimization that automatically "possessifies" certain simple pattern constructs. For example, the sequence A+B is treated as A++B because there is no point in backtracking into a sequence of A's when B must follow. This feature can be disabled by the PCRE2_NO_AUTO- POSSESS option, or starting the pattern with (*NO_AUTO_POSSESS).
When a pattern contains an unlimited repeat inside a group that can it- self be repeated an unlimited number of times, the use of an atomic group is the only way to avoid some failing matches taking a very long time indeed. The pattern
(\D+|<\d+>)*[!?]
matches an unlimited number of substrings that either consist of non- digits, or digits enclosed in <>, followed by either ! or ?. When it matches, it runs quickly. However, if it is applied to
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
it takes a long time before reporting failure. This is because the string can be divided between the internal \D+ repeat and the external * repeat in a large number of ways, and all have to be tried. (The ex- ample uses [!?] rather than a single character at the end, because both PCRE2 and Perl have an optimization that allows for fast failure when a single character is used. They remember the last single character that is required for a match, and fail early if it is not present in the string.) If the pattern is changed so that it uses an atomic group, like this:
((?>\D+)|<\d+>)*[!?]
sequences of non-digits cannot be broken, and failure happens quickly.
BACKREFERENCES
Outside a character class, a backslash followed by a digit greater than 0 (and possibly further digits) is a backreference to a capture group earlier (that is, to its left) in the pattern, provided there have been that many previous capture groups.
However, if the decimal number following the backslash is less than 8, it is always taken as a backreference, and causes an error only if there are not that many capture groups in the entire pattern. In other words, the group that is referenced need not be to the left of the ref- erence for numbers less than 8. A "forward backreference" of this type can make sense when a repetition is involved and the group to the right has participated in an earlier iteration.
It is not possible to have a numerical "forward backreference" to a group whose number is 8 or more using this syntax because a sequence such as \50 is interpreted as a character defined in octal. See the subsection entitled "Non-printing characters" above for further details of the handling of digits following a backslash. Other forms of back- referencing do not suffer from this restriction. In particular, there is no problem when named capture groups are used (see below).
Another way of avoiding the ambiguity inherent in the use of digits following a backslash is to use the \g escape sequence. This escape must be followed by a signed or unsigned number, optionally enclosed in braces. These examples are all identical:
(ring), \1 (ring), \g1 (ring), \g{1}
An unsigned number specifies an absolute reference without the ambigu- ity that is present in the older syntax. It is also useful when literal digits follow the reference. A signed number is a relative reference. Consider this example:
(abc(def)ghi)\g{-1}
The sequence \g{-1} is a reference to the capture group whose number is one less than the number of the next group to be started, so in this example (where the next group would be numbered 3) is it equivalent to \2, and \g{-2} would be equivalent to \1. Note that if this construct is inside a capture group, that group is included in the count, so in this example \g{-2} also refers to group 1:
(A)(\g{-2}B)
The use of relative references can be helpful in long patterns, and also in patterns that are created by joining together fragments that contain references within themselves.
The sequence \g{+1} is a reference to the next capture group that is started after this item, and \g{+2} refers to the one after that, and so on. This kind of forward reference can be useful in patterns that repeat. Perl does not support the use of + in this way.
A backreference matches whatever actually most recently matched the capture group in the current subject string, rather than anything at all that matches the group (see "Groups as subroutines" below for a way of doing that). So the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility". If caseful matching is in force at the time of the backreference, the case of letters is relevant. For exam- ple,
((?i)rah)\s+\1
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original capture group is matched caselessly.
There are several different ways of writing backreferences to named capture groups. The .NET syntax is \k{name}, the Python syntax is (?=name), and the original Perl syntax is \k<name> or \k'name'. All of these are now supported by both Perl and PCRE2. Perl 5.10's unified backreference syntax, in which \g can be used for both numeric and named references, is also supported by PCRE2. We could rewrite the above example in any of the following ways:
(?<p1>(?i)rah)\s+\k<p1> (?'p1'(?i)rah)\s+\k{p1} (?P<p1>(?i)rah)\s+(?P=p1) (?<p1>(?i)rah)\s+\g{p1}
A capture group that is referenced by name may appear in the pattern before or after the reference.
There may be more than one backreference to the same group. If a group has not actually been used in a particular match, backreferences to it always fail by default. For example, the pattern
(a|(bc))\2
always fails if it starts to match "a" rather than "bc". However, if the PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a backref- erence to an unset value matches an empty string.
Because there may be many capture groups in a pattern, all digits fol- lowing a backslash are taken as part of a potential backreference num- ber. If the pattern continues with a digit character, some delimiter must be used to terminate the backreference. If the PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, this can be white space. Otherwise, the \g{} syntax or an empty comment (see "Comments" below) can be used.
Recursive backreferences
A backreference that occurs inside the group to which it refers fails when the group is first used, so, for example, (a\1) never matches. However, such references can be useful inside repeated groups. For ex- ample, the pattern
(a|b\1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- ation of the group, the backreference matches the character string cor- responding to the previous iteration. In order for this to work, the pattern must be such that the first iteration does not need to match the backreference. This can be done using alternation, as in the exam- ple above, or by a quantifier with a minimum of zero.
For versions of PCRE2 less than 10.25, backreferences of this type used to cause the group that they reference to be treated as an atomic group. This restriction no longer applies, and backtracking into such groups can occur as normal.
ASSERTIONS
An assertion is a test on the characters following or preceding the current matching point that does not consume any characters. The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described above.
More complicated assertions are coded as parenthesized groups. There are two kinds: those that look ahead of the current position in the subject string, and those that look behind it, and in each case an as- sertion may be positive (must match for the assertion to be true) or negative (must not match for the assertion to be true). An assertion group is matched in the normal way, and if it is true, matching contin- ues after it, but with the matching position in the subject string re- set to what it was before the assertion was processed.
The Perl-compatible lookaround assertions are atomic. If an assertion is true, but there is a subsequent matching failure, there is no back- tracking into the assertion. However, there are some cases where non- atomic assertions can be useful. PCRE2 has some support for these, de- scribed in the section entitled "Non-atomic assertions" below, but they are not Perl-compatible.
A lookaround assertion may appear as the condition in a conditional group (see below). In this case, the result of matching the assertion determines which branch of the condition is followed.
Assertion groups are not capture groups. If an assertion contains cap- ture groups within it, these are counted for the purposes of numbering the capture groups in the whole pattern. Within each branch of an as- sertion, locally captured substrings may be referenced in the usual way. For example, a sequence such as (.)\g{-1} can be used to check that two adjacent characters are the same.
When a branch within an assertion fails to match, any substrings that were captured are discarded (as happens with any pattern branch that fails to match). A negative assertion is true only when all its branches fail to match; this means that no captured substrings are ever retained after a successful negative assertion. When an assertion con- tains a matching branch, what happens depends on the type of assertion.
For a positive assertion, internally captured substrings in the suc- cessful branch are retained, and matching continues with the next pat- tern item after the assertion. For a negative assertion, a matching branch means that the assertion is not true. If such an assertion is being used as a condition in a conditional group (see below), captured substrings are retained, because matching continues with the "no" branch of the condition. For other failing negative assertions, control passes to the previous backtracking point, thus discarding any captured strings within the assertion.
Most assertion groups may be repeated; though it makes no sense to as- sert the same thing several times, the side effect of capturing in pos- itive assertions may occasionally be useful. However, an assertion that forms the condition for a conditional group may not be quantified. PCRE2 used to restrict the repetition of assertions, but from release 10.35 the only restriction is that an unlimited maximum repetition is changed to be one more than the minimum. For example, {3,} is treated as {3,4}.
Alphabetic assertion names
Traditionally, symbolic sequences such as (?= and (?<= have been used to specify lookaround assertions. Perl 5.28 introduced some experimen- tal alphabetic alternatives which might be easier to remember. They all start with (* instead of (? and must be written using lower case let- ters. PCRE2 supports the following synonyms:
(*positive_lookahead: or (*pla: is the same as (?= (*negative_lookahead: or (*nla: is the same as (?! (*positive_lookbehind: or (*plb: is the same as (?<= (*negative_lookbehind: or (*nlb: is the same as (?<!
For example, (*pla:foo) is the same assertion as (?=foo). In the fol- lowing sections, the various assertions are described using the origi- nal symbolic forms.
Lookahead assertions
Lookahead assertions start with (?= for positive assertions and (?! for negative assertions. For example,
\w+(?=;)
matches a word followed by a semicolon, but does not include the semi- colon in the match, and
foo(?!bar)
matches any occurrence of "foo" that is not followed by "bar". Note that the apparently similar pattern
(?!foo)bar
does not find an occurrence of "bar" that is preceded by something other than "foo"; it finds any occurrence of "bar" whatsoever, because the assertion (?!foo) is always true when the next three characters are "bar". A lookbehind assertion is needed to achieve the other effect.
If you want to force a matching failure at some point in a pattern, the most convenient way to do it is with (?!) because an empty string al- ways matches, so an assertion that requires there not to be an empty string must always fail. The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
Lookbehind assertions
Lookbehind assertions start with (?<= for positive assertions and (?<! for negative assertions. For example,
(?<!foo)bar
does find an occurrence of "bar" that is not preceded by "foo". The contents of a lookbehind assertion are restricted such that there must be a known maximum to the lengths of all the strings it matches. There are two cases:
If every top-level alternative matches a fixed length, for example
(?<=colour|color)
there is a limit of 65535 characters to the lengths, which do not have to be the same, as this example demonstrates. This is the only kind of lookbehind supported by PCRE2 versions earlier than 10.43 and by the alternative matching function pcre2_dfa_match().
In PCRE2 10.43 and later, pcre2_match() supports lookbehind assertions in which one or more top-level alternatives can match more than one string length, for example
(?<=colou?r)
The maximum matching length for any branch of the lookbehind is limited to a value set by the calling program (default 255 characters). Unlim- ited repetition (for example \d*) is not supported. In some cases, the escape sequence \K (see above) can be used instead of a lookbehind as- sertion at the start of a pattern to get round the length limit re- striction.
In UTF-8 and UTF-16 modes, PCRE2 does not allow the \C escape (which matches a single code unit even in a UTF mode) to appear in lookbehind assertions, because it makes it impossible to calculate the length of the lookbehind. The \X and \R escapes, which can match different num- bers of code units, are never permitted in lookbehinds.
"Subroutine" calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long as the called capture group matches a limited- length string. However, recursion, that is, a "subroutine" call into a group that is already active, is not supported.
PCRE2 supports backreferences in lookbehinds, but only if certain con- ditions are met. The PCRE2_MATCH_UNSET_BACKREF option must not be set, there must be no use of (?| in the pattern (it creates duplicate group numbers), and if the backreference is by name, the name must be unique. Of course, the referenced group must itself match a limited length sub- string. The following pattern matches words containing at least two characters that begin and end with the same character:
\b(\w)\w++(?<=\1)
Possessive quantifiers can be used in conjunction with lookbehind as- sertions to specify efficient matching at the end of subject strings. Consider a simple pattern such as
abcd$
when applied to a long string that does not match. Because matching proceeds from left to right, PCRE2 will look for each "a" in the sub- ject and then see if what follows matches the rest of the pattern. If the pattern is specified as
^.*abcd$
the initial .* matches the entire string at first, but when this fails (because there is no following "a"), it backtracks to match all but the last character, then all but the last two characters, and so on. Once again the search for "a" covers the entire string, from right to left, so we are no better off. However, if the pattern is written as
^.*+(?<=abcd)
there can be no backtracking for the .*+ item because of the possessive quantifier; it can match only the entire string. The subsequent lookbe- hind assertion does a single test on the last four characters. If it fails, the match fails immediately. For long strings, this approach makes a significant difference to the processing time.
Using multiple assertions
Several assertions (of any sort) may occur in succession. For example,
(?<=\d{3})(?<!999)foo
matches "foo" preceded by three digits that are not "999". Notice that each of the assertions is applied independently at the same point in the subject string. First there is a check that the previous three characters are all digits, and then there is a check that the same three characters are not "999". This pattern does not match "foo" pre- ceded by six characters, the first of which are digits and the last three of which are not "999". For example, it doesn't match "123abc- foo". A pattern to do that is
(?<=\d{3}...)(?<!999)foo
This time the first assertion looks at the preceding six characters, checking that the first three are digits, and then the second assertion checks that the preceding three characters are not "999".
Assertions can be nested in any combination. For example,
(?<=(?<!foo)bar)baz
matches an occurrence of "baz" that is preceded by "bar" which in turn is not preceded by "foo", while
(?<=\d{3}(?!999)...)foo
is another pattern that matches "foo" preceded by three digits and any three characters that are not "999".
NON-ATOMIC ASSERTIONS
Traditional lookaround assertions are atomic. That is, if an assertion is true, but there is a subsequent matching failure, there is no back- tracking into the assertion. However, there are some cases where non- atomic positive assertions can be useful. PCRE2 provides these using the following syntax:
(*non_atomic_positive_lookahead: or (*napla: or (?* (*non_atomic_positive_lookbehind: or (*naplb: or (?<*
Consider the problem of finding the right-most word in a string that also appears earlier in the string, that is, it must appear at least twice in total. This pattern returns the required result as captured substring 1:
^(?x)(*napla: .* \b(\w++)) (?> .*? \b\1\b ){2}
For a subject such as "word1 word2 word3 word2 word3 word4" the result is "word3". How does it work? At the start, ^(?x) anchors the pattern and sets the "x" option, which causes white space (introduced for read- ability) to be ignored. Inside the assertion, the greedy .* at first consumes the entire string, but then has to backtrack until the rest of the assertion can match a word, which is captured by group 1. In other words, when the assertion first succeeds, it captures the right-most word in the string.
The current matching point is then reset to the start of the subject, and the rest of the pattern match checks for two occurrences of the captured word, using an ungreedy .*? to scan from the left. If this succeeds, we are done, but if the last word in the string does not oc- cur twice, this part of the pattern fails. If a traditional atomic lookahead (?= or (*pla: had been used, the assertion could not be re- entered, and the whole match would fail. The pattern would succeed only if the very last word in the subject was found twice.
Using a non-atomic lookahead, however, means that when the last word does not occur twice in the string, the lookahead can backtrack and find the second-last word, and so on, until either the match succeeds, or all words have been tested.
Two conditions must be met for a non-atomic assertion to be useful: the contents of one or more capturing groups must change after a backtrack into the assertion, and there must be a backreference to a changed group later in the pattern. If this is not the case, the rest of the pattern match fails exactly as before because nothing has changed, so using a non-atomic assertion just wastes resources.
There is one exception to backtracking into a non-atomic assertion. If an (*ACCEPT) control verb is triggered, the assertion succeeds atomi- cally. That is, a subsequent match failure cannot backtrack into the assertion.
Non-atomic assertions are not supported by the alternative matching function pcre2_dfa_match(). They are supported by JIT, but only if they do not contain any control verbs such as (*ACCEPT). (This may change in future). Note that assertions that appear as conditions for conditional groups (see below) must be atomic.
SCRIPT RUNS
In concept, a script run is a sequence of characters that are all from the same Unicode script such as Latin or Greek. However, because some scripts are commonly used together, and because some diacritical and other marks are used with multiple scripts, it is not that simple. There is a full description of the rules that PCRE2 uses in the section entitled "Script Runs" in the pcre2unicode documentation.
If part of a pattern is enclosed between (*script_run: or (*sr: and a closing parenthesis, it fails if the sequence of characters that it matches are not a script run. After a failure, normal backtracking oc- curs. Script runs can be used to detect spoofing attacks using charac- ters that look the same, but are from different scripts. The string "paypal.com" is an infamous example, where the letters could be a mix- ture of Latin and Cyrillic. This pattern ensures that the matched char- acters in a sequence of non-spaces that follow white space are a script run:
\s+(*sr:\S+)
To be sure that they are all from the Latin script (for example), a lookahead can be used:
\s+(?=\p{Latin})(*sr:\S+)
This works as long as the first character is expected to be a character in that script, and not (for example) punctuation, which is allowed with any script. If this is not the case, a more creative lookahead is needed. For example, if digits, underscore, and dots are permitted at the start:
\s+(?=[0-9_.]*\p{Latin})(*sr:\S+)
In many cases, backtracking into a script run pattern fragment is not desirable. The script run can employ an atomic group to prevent this. Because this is a common requirement, a shorthand notation is provided by (*atomic_script_run: or (*asr:
(*asr:...) is the same as (*sr:(?>...))
Note that the atomic group is inside the script run. Putting it outside would not prevent backtracking into the script run pattern.
Support for script runs is not available if PCRE2 is compiled without Unicode support. A compile-time error is given if any of the above con- structs is encountered. Script runs are not supported by the alternate matching function, pcre2_dfa_match() because they use the same mecha- nism as capturing parentheses.
Warning: The (*ACCEPT) control verb (see below) should not be used within a script run group, because it causes an immediate exit from the group, bypassing the script run checking.
CONDITIONAL GROUPS
It is possible to cause the matching process to obey a pattern fragment conditionally or to choose between two alternative fragments, depending on the result of an assertion, or whether a specific capture group has already been matched. The two possible forms of conditional group are:
(?(condition)yes-pattern) (?(condition)yes-pattern|no-pattern)
If the condition is satisfied, the yes-pattern is used; otherwise the no-pattern (if present) is used. An absent no-pattern is equivalent to an empty string (it always matches). If there are more than two alter- natives in the group, a compile-time error occurs. Each of the two al- ternatives may itself contain nested groups of any form, including con- ditional groups; the restriction to two alternatives applies only at the level of the condition itself. This pattern fragment is an example where the alternatives are complex:
(?(1) (A|B|C) | (D | (?(2)E|F) | E) )
There are five kinds of condition: references to capture groups, refer- ences to recursion, two pseudo-conditions called DEFINE and VERSION, and assertions.
Checking for a used capture group by number
If the text between the parentheses consists of a sequence of digits, the condition is true if a capture group of that number has previously matched. If there is more than one capture group with the same number (see the earlier section about duplicate group numbers), the condition is true if any of them have matched. An alternative notation, which is a PCRE2 extension, not supported by Perl, is to precede the digits with a plus or minus sign. In this case, the group number is relative rather than absolute. The most recently opened capture group (which could be enclosing this condition) can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside loops it can also make sense to re- fer to subsequent groups. The next capture group to be opened can be referenced as (?(+1), and so on. The value zero in any of these forms is not used; it provokes a compile-time error.
Consider the following pattern, which contains non-significant white space to make it more readable (assume the PCRE2_EXTENDED option) and to divide it into three parts for ease of discussion:
( \( )? [^()]+ (?(1) \) )
The first part matches an optional opening parenthesis, and if that character is present, sets it as the first captured substring. The sec- ond part matches one or more characters that are not parentheses. The third part is a conditional group that tests whether or not the first capture group matched. If it did, that is, if subject started with an opening parenthesis, the condition is true, and so the yes-pattern is executed and a closing parenthesis is required. Otherwise, since no- pattern is not present, the conditional group matches nothing. In other words, this pattern matches a sequence of non-parentheses, optionally enclosed in parentheses.
If you were embedding this pattern in a larger one, you could use a relative reference:
...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
This makes the fragment independent of the parentheses in the larger pattern.
Checking for a used capture group by name
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used capture group by name. For compatibility with earlier versions of PCRE1, which had this facility before Perl, the syntax (?(name)...) is also recognized. Note, however, that undelimited names consisting of the letter R followed by digits are ambiguous (see the following sec- tion). Rewriting the above example to use a named group gives this:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
If the name used in a condition of this kind is a duplicate, the test is applied to all groups of the same name, and is true if any one of them has matched.
Checking for pattern recursion
"Recursion" in this sense refers to any subroutine-like call from one part of the pattern to another, whether or not it is actually recur- sive. See the sections entitled "Recursive patterns" and "Groups as subroutines" below for details of recursion and subroutine calls.
If a condition is the string (R), and there is no capture group with the name R, the condition is true if matching is currently in a recur- sion or subroutine call to the whole pattern or any capture group. If digits follow the letter R, and there is no group with that name, the condition is true if the most recent call is into a group with the given number, which must exist somewhere in the overall pattern. This is a contrived example that is equivalent to a+b:
((?(R1)a+|(?1)b))
However, in both cases, if there is a capture group with a matching name, the condition tests for its being set, as described in the sec- tion above, instead of testing for recursion. For example, creating a group with the name R1 by adding (?<R1>) to the above pattern com- pletely changes its meaning.
If a name preceded by ampersand follows the letter R, for example:
(?(R&name)...)
the condition is true if the most recent recursion is into a group of that name (which must exist within the pattern).
This condition does not check the entire recursion stack. It tests only the current level. If the name used in a condition of this kind is a duplicate, the test is applied to all groups of the same name, and is true if any one of them is the most recent recursion.
At "top level", all these recursion test conditions are false.
Defining capture groups for use by reference only
If the condition is the string (DEFINE), the condition is always false, even if there is a group with the name DEFINE. In this case, there may be only one alternative in the rest of the conditional group. It is al- ways skipped if control reaches this point in the pattern; the idea of DEFINE is that it can be used to define subroutines that can be refer- enced from elsewhere. (The use of subroutines is described below.) For example, a pattern to match an IPv4 address such as "192.168.23.245" could be written like this (ignore white space and line breaks):
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) \b (?&byte) (\.(?&byte)){3} \b
The first part of the pattern is a DEFINE group inside which another group named "byte" is defined. This matches an individual component of an IPv4 address (a number less than 256). When matching takes place, this part of the pattern is skipped because DEFINE acts like a false condition. The rest of the pattern uses references to the named group to match the four dot-separated components of an IPv4 address, insist- ing on a word boundary at each end.
Checking the PCRE2 version
Programs that link with a PCRE2 library can check the version by call- ing pcre2_config() with appropriate arguments. Users of applications that do not have access to the underlying code cannot do this. A spe- cial "condition" called VERSION exists to allow such users to discover which version of PCRE2 they are dealing with by using this condition to match a string such as "yesno". VERSION must be followed either by "=" or ">=" and a version number. For example:
(?(VERSION>=10.4)yes|no)
This pattern matches "yes" if the PCRE2 version is greater or equal to 10.4, or "no" otherwise. The fractional part of the version number may not contain more than two digits.
Assertion conditions
If the condition is not in any of the above formats, it must be a parenthesized assertion. This may be a positive or negative lookahead or lookbehind assertion. However, it must be a traditional atomic as- sertion, not one of the non-atomic assertions.
Consider this pattern, again containing non-significant white space, and with the two alternatives on the second line:
(?(?=[^a-z]*[a-z]) \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
The condition is a positive lookahead assertion that matches an op- tional sequence of non-letters followed by a letter. In other words, it tests for the presence of at least one letter in the subject. If a let- ter is found, the subject is matched against the first alternative; otherwise it is matched against the second. This pattern matches strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
When an assertion that is a condition contains capture groups, any cap- turing that occurs in a matching branch is retained afterwards, for both positive and negative assertions, because matching always contin- ues after the assertion, whether it succeeds or fails. (Compare non- conditional assertions, for which captures are retained only for posi- tive assertions that succeed.)
COMMENTS
There are two ways of including comments in patterns that are processed by PCRE2. In both cases, the start of the comment must not be in a character class, nor in the middle of any other sequence of related characters such as (?: or a group name or number. The characters that make up a comment play no part in the pattern matching.
The sequence (?# marks the start of a comment that continues up to the next closing parenthesis. Nested parentheses are not permitted. If the PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, an unescaped # character also introduces a comment, which in this case continues to immediately after the next newline character or character sequence in the pattern. Which characters are interpreted as newlines is controlled by an option passed to the compiling function or by a special sequence at the start of the pattern, as described in the section entitled "New- line conventions" above. Note that the end of this type of comment is a literal newline sequence in the pattern; escape sequences that happen to represent a newline do not count. For example, consider this pattern when PCRE2_EXTENDED is set, and the default newline convention (a sin- gle linefeed character) is in force:
abc #comment \n still comment
On encountering the # character, pcre2_compile() skips along, looking for a newline in the pattern. The sequence \n is still literal at this stage, so it does not terminate the comment. Only an actual character with the code value 0x0a (the default newline) does so.
RECURSIVE PATTERNS
Consider the problem of matching a string in parentheses, allowing for unlimited nested parentheses. Without the use of recursion, the best that can be done is to use a pattern that matches up to some fixed depth of nesting. It is not possible to handle an arbitrary nesting depth.
For some time, Perl has provided a facility that allows regular expres- sions to recurse (amongst other things). It does this by interpolating Perl code in the expression at run time, and the code can refer to the expression itself. A Perl pattern using code interpolation to solve the parentheses problem can be created like this:
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
The (?p{...}) item interpolates Perl code at run time, and in this case refers recursively to the pattern in which it appears.
Obviously, PCRE2 cannot support the interpolation of Perl code. In- stead, it supports special syntax for recursion of the entire pattern, and also for individual capture group recursion. After its introduction in PCRE1 and Python, this kind of recursion was subsequently introduced into Perl at release 5.10.
A special item that consists of (? followed by a number greater than zero and a closing parenthesis is a recursive subroutine call of the capture group of the given number, provided that it occurs inside that group. (If not, it is a non-recursive subroutine call, which is de- scribed in the next section.) The special item (?R) or (?0) is a recur- sive call of the entire regular expression.
This PCRE2 pattern solves the nested parentheses problem (assume the PCRE2_EXTENDED option is set so that white space is ignored):
\( ( [^()]++ | (?R) )* \)
First it matches an opening parenthesis. Then it matches any number of substrings which can either be a sequence of non-parentheses, or a re- cursive match of the pattern itself (that is, a correctly parenthesized substring). Finally there is a closing parenthesis. Note the use of a possessive quantifier to avoid backtracking into sequences of non- parentheses.
If this were part of a larger pattern, you would not want to recurse the entire pattern, so instead you could use this:
( \( ( [^()]++ | (?1) )* \) )
We have put the pattern into parentheses, and caused the recursion to refer to them instead of the whole pattern.
In a larger pattern, keeping track of parenthesis numbers can be tricky. This is made easier by the use of relative references. Instead of (?1) in the pattern above you can write (?-2) to refer to the second most recently opened parentheses preceding the recursion. In other words, a negative number counts capturing parentheses leftwards from the point at which it is encountered.
Be aware however, that if duplicate capture group numbers are in use, relative references refer to the earliest group with the appropriate number. Consider, for example:
(?|(a)|(b)) (c) (?-2)
The first two capture groups (a) and (b) are both numbered 1, and group (c) is number 2. When the reference (?-2) is encountered, the second most recently opened parentheses has the number 1, but it is the first such group (the (a) group) to which the recursion refers. This would be the same if an absolute reference (?1) was used. In other words, rela- tive references are just a shorthand for computing a group number.
It is also possible to refer to subsequent capture groups, by writing references such as (?+2). However, these cannot be recursive because the reference is not inside the parentheses that are referenced. They are always non-recursive subroutine calls, as described in the next section.
An alternative approach is to use named parentheses. The Perl syntax for this is (?&name); PCRE1's earlier syntax (?P>name) is also sup- ported. We could rewrite the above example as follows:
(?<pn> \( ( [^()]++ | (?&pn) )* \) )
If there is more than one group with the same name, the earliest one is used.
The example pattern that we have been looking at contains nested unlim- ited repeats, and so the use of a possessive quantifier for matching strings of non-parentheses is important when applying the pattern to strings that do not match. For example, when this pattern is applied to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
it yields "no match" quickly. However, if a possessive quantifier is not used, the match runs for a very long time indeed because there are so many different ways the + and * repeats can carve up the subject, and all have to be tested before failure can be reported.
At the end of a match, the values of capturing parentheses are those from the outermost level. If you want to obtain intermediate values, a callout function can be used (see below and the pcre2callout documenta- tion). If the pattern above is matched against
(ab(cd)ef)
the value for the inner capturing parentheses (numbered 2) is "ef", which is the last value taken on at the top level. If a capture group is not matched at the top level, its final captured value is unset, even if it was (temporarily) set at a deeper level during the matching process.
Do not confuse the (?R) item with the condition (R), which tests for recursion. Consider this pattern, which matches text in angle brack- ets, allowing for arbitrary nesting. Only digits are allowed in nested brackets (that is, when recursing), whereas any characters are permit- ted at the outer level.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
In this pattern, (?(R) is the start of a conditional group, with two different alternatives for the recursive and non-recursive cases. The (?R) item is the actual recursive call.
Differences in recursion processing between PCRE2 and Perl
Some former differences between PCRE2 and Perl no longer exist.
Before release 10.30, recursion processing in PCRE2 differed from Perl in that a recursive subroutine call was always treated as an atomic group. That is, once it had matched some of the subject string, it was never re-entered, even if it contained untried alternatives and there was a subsequent matching failure. (Historical note: PCRE implemented recursion before Perl did.)
Starting with release 10.30, recursive subroutine calls are no longer treated as atomic. That is, they can be re-entered to try unused alter- natives if there is a matching failure later in the pattern. This is now compatible with the way Perl works. If you want a subroutine call to be atomic, you must explicitly enclose it in an atomic group.
Supporting backtracking into recursions simplifies certain types of re- cursive pattern. For example, this pattern matches palindromic strings:
^((.)(?1)\2|.?)$
The second branch in the group matches a single central character in the palindrome when there are an odd number of characters, or nothing when there are an even number of characters, but in order to work it has to be able to try the second case when the rest of the pattern match fails. If you want to match typical palindromic phrases, the pat- tern has to ignore all non-word characters, which can be done like this:
^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$
If run with the PCRE2_CASELESS option, this pattern matches phrases such as "A man, a plan, a canal: Panama!". Note the use of the posses- sive quantifier *+ to avoid backtracking into sequences of non-word characters. Without this, PCRE2 takes a great deal longer (ten times or more) to match typical phrases, and Perl takes so long that you think it has gone into a loop.
Another way in which PCRE2 and Perl used to differ in their recursion processing is in the handling of captured values. Formerly in Perl, when a group was called recursively or as a subroutine (see the next section), it had no access to any values that were captured outside the recursion, whereas in PCRE2 these values can be referenced. Consider this pattern:
^(.)(\1|a(?2))
This pattern matches "bab". The first capturing parentheses match "b", then in the second group, when the backreference \1 fails to match "b", the second alternative matches "a" and then recurses. In the recursion, \1 does now match "b" and so the whole match succeeds. This match used to fail in Perl, but in later versions (I tried 5.024) it now works.
GROUPS AS SUBROUTINES
If the syntax for a recursive group call (either by number or by name) is used outside the parentheses to which it refers, it operates a bit like a subroutine in a programming language. More accurately, PCRE2 treats the referenced group as an independent subpattern which it tries to match at the current matching position. The called group may be de- fined before or after the reference. A numbered reference can be ab- solute or relative, as in these examples:
(...(absolute)...)...(?2)... (...(relative)...)...(?-1)... (...(?+1)...(relative)...
An earlier example pointed out that the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility". If instead the pattern
(sens|respons)e and (?1)ibility
is used, it does match "sense and responsibility" as well as the other two strings. Another example is given in the discussion of DEFINE above.
Like recursions, subroutine calls used to be treated as atomic, but this changed at PCRE2 release 10.30, so backtracking into subroutine calls can now occur. However, any capturing parentheses that are set during the subroutine call revert to their previous values afterwards.
Processing options such as case-independence are fixed when a group is defined, so if it is used as a subroutine, such options cannot be changed for different calls. For example, consider this pattern:
(abc)(?i:(?-1))
It matches "abcabc". It does not match "abcABC" because the change of processing option does not affect the called group.
The behaviour of backtracking control verbs in groups when called as subroutines is described in the section entitled "Backtracking verbs in subroutines" below.
ONIGURUMA SUBROUTINE SYNTAX
For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either in angle brackets or single quotes, is an alternative syntax for calling a group as a subroutine, possibly re- cursively. Here are two of the examples used above, rewritten using this syntax:
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) (sens|respons)e and \g'1'ibility
PCRE2 supports an extension to Oniguruma: if a number is preceded by a plus or a minus sign it is taken as a relative reference. For example:
(abc)(?i:\g<-1>)
Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former is a backreference; the latter is a subroutine call.
CALLOUTS
Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl code to be obeyed in the middle of matching a regular expression. This makes it possible, amongst other things, to extract different sub- strings that match the same pair of parentheses when there is a repeti- tion.
PCRE2 provides a similar feature, but of course it cannot obey arbi- trary Perl code. The feature is called "callout". The caller of PCRE2 provides an external function by putting its entry point in a match context using the function pcre2_set_callout(), and then passing that context to pcre2_match() or pcre2_dfa_match(). If no match context is passed, or if the callout entry point is set to NULL, callouts are dis- abled.
Within a regular expression, (?C<arg>) indicates a point at which the external function is to be called. There are two kinds of callout: those with a numerical argument and those with a string argument. (?C) on its own with no argument is treated as (?C0). A numerical argument allows the application to distinguish between different callouts. String arguments were added for release 10.20 to make it possible for script languages that use PCRE2 to embed short scripts within patterns in a similar way to Perl.
During matching, when PCRE2 reaches a callout point, the external func- tion is called. It is provided with the number or string argument of the callout, the position in the pattern, and one item of data that is also set in the match block. The callout function may cause matching to proceed, to backtrack, or to fail.
By default, PCRE2 implements a number of optimizations at matching time, and one side-effect is that sometimes callouts are skipped. If you need all possible callouts to happen, you need to set options that disable the relevant optimizations. More details, including a complete description of the programming interface to the callout function, are given in the pcre2callout documentation.
Callouts with numerical arguments
If you just want to have a means of identifying different callout points, put a number less than 256 after the letter C. For example, this pattern has two callout points:
(?C1)abc(?C2)def
If the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical callouts are automatically installed before each item in the pattern. They are all numbered 255. If there is a conditional group in the pat- tern whose condition is an assertion, an additional callout is inserted just before the condition. An explicit callout may also be set at this position, as in this example:
(?(?C9)(?=a)abc|def)
Note that this applies only to assertion conditions, not to other types of condition.
Callouts with string arguments
A delimited string may be used instead of a number as a callout argu- ment. The starting delimiter must be one of ` ' " ^ % # $ { and the ending delimiter is the same as the start, except for {, where the end- ing delimiter is }. If the ending delimiter is needed within the string, it must be doubled. For example:
(?C'ab ''c'' d')xyz(?C{any text})pqr
The doubling is removed before the string is passed to the callout function.
BACKTRACKING CONTROL
There are a number of special "Backtracking Control Verbs" (to use Perl's terminology) that modify the behaviour of backtracking during matching. They are generally of the form (*VERB) or (*VERB:NAME). Some verbs take either form, and may behave differently depending on whether or not a name argument is present. The names are not required to be unique within the pattern.
By default, for compatibility with Perl, a name is any sequence of characters that does not include a closing parenthesis. The name is not processed in any way, and it is not possible to include a closing parenthesis in the name. This can be changed by setting the PCRE2_ALT_VERBNAMES option, but the result is no longer Perl-compati- ble.
When PCRE2_ALT_VERBNAMES is set, backslash processing is applied to verb names and only an unescaped closing parenthesis terminates the name. However, the only backslash items that are permitted are \Q, \E, and sequences such as \x{100} that define character code points. Char- acter type escapes such as \d are faulted.
A closing parenthesis can be included in a name either as \) or between \Q and \E. In addition to backslash processing, if the PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is also set, unescaped whitespace in verb names is skipped, and #-comments are recognized, exactly as in the rest of the pattern. PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not affect verb names unless PCRE2_ALT_VERBNAMES is also set.
The maximum length of a name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit libraries. If the name is empty, that is, if the closing parenthesis immediately follows the colon, the effect is as if the colon were not there. Any number of these verbs may occur in a pat- tern. Except for (*ACCEPT), they may not be quantified.
Since these verbs are specifically related to backtracking, most of them can be used only when the pattern is to be matched using the tra- ditional matching function, because that uses a backtracking algorithm. With the exception of (*FAIL), which behaves like a failing negative assertion, the backtracking control verbs cause an error if encountered by the DFA matching function.
The behaviour of these verbs in repeated groups, assertions, and in capture groups called as subroutines (whether or not recursively) is documented below.
Optimizations that affect backtracking verbs
PCRE2 contains some optimizations that are used to speed up matching by running some checks at the start of each match attempt. For example, it may know the minimum length of matching subject, or that a particular character must be present. When one of these optimizations bypasses the running of a match, any included backtracking verbs will not, of course, be processed. You can suppress the start-of-match optimizations by setting the PCRE2_NO_START_OPTIMIZE option when calling pcre2_com- pile(), or by starting the pattern with (*NO_START_OPT). There is more discussion of this option in the section entitled "Compiling a pattern" in the pcre2api documentation.
Experiments with Perl suggest that it too has similar optimizations, and like PCRE2, turning them off can change the result of a match.
Verbs that act immediately
The following verbs act as soon as they are encountered.
(*ACCEPT) or (*ACCEPT:NAME)
This verb causes the match to end successfully, skipping the remainder of the pattern. However, when it is inside a capture group that is called as a subroutine, only that group is ended successfully. Matching then continues at the outer level. If (*ACCEPT) in triggered in a posi- tive assertion, the assertion succeeds; in a negative assertion, the assertion fails.
If (*ACCEPT) is inside capturing parentheses, the data so far is cap- tured. For example:
A((?:A|B(*ACCEPT)|C)D)
This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap- tured by the outer parentheses.
(*ACCEPT) is the only backtracking verb that is allowed to be quanti- fied because an ungreedy quantification with a minimum of zero acts only when a backtrack happens. Consider, for example,
(A(*ACCEPT)??B)C
where A, B, and C may be complex expressions. After matching "A", the matcher processes "BC"; if that fails, causing a backtrack, (*ACCEPT) is triggered and the match succeeds. In both cases, all but C is cap- tured. Whereas (*COMMIT) (see below) means "fail on backtrack", a re- peated (*ACCEPT) of this type means "succeed on backtrack".
Warning: (*ACCEPT) should not be used within a script run group, be- cause it causes an immediate exit from the group, bypassing the script run checking.
(*FAIL) or (*FAIL:NAME)
This verb causes a matching failure, forcing backtracking to occur. It may be abbreviated to (*F). It is equivalent to (?!) but easier to read. The Perl documentation notes that it is probably useful only when combined with (?{}) or (??{}). Those are, of course, Perl features that are not present in PCRE2. The nearest equivalent is the callout fea- ture, as for example in this pattern:
a+(?C)(*FAIL)
A match with the string "aaaa" always fails, but the callout is taken before each backtrack happens (in this example, 10 times).
(*ACCEPT:NAME) and (*FAIL:NAME) behave the same as (*MARK:NAME)(*AC- CEPT) and (*MARK:NAME)(*FAIL), respectively, that is, a (*MARK) is recorded just before the verb acts.
Recording which path was taken
There is one verb whose main purpose is to track how a match was ar- rived at, though it also has a secondary use in conjunction with ad- vancing the match starting point (see (*SKIP) below).
(*MARK:NAME) or (*:NAME)
A name is always required with this verb. For all the other backtrack- ing control verbs, a NAME argument is optional.
When a match succeeds, the name of the last-encountered mark name on the matching path is passed back to the caller as described in the sec- tion entitled "Other information about the match" in the pcre2api docu- mentation. This applies to all instances of (*MARK) and other verbs, including those inside assertions and atomic groups. However, there are differences in those cases when (*MARK) is used in conjunction with (*SKIP) as described below.
The mark name that was last encountered on the matching path is passed back. A verb without a NAME argument is ignored for this purpose. Here is an example of pcre2test output, where the "mark" modifier requests the retrieval and outputting of (*MARK) data:
re> /X(*MARK:A)Y|X(*MARK:B)Z/mark data> XY 0: XY MK: A XZ 0: XZ MK: B
The (*MARK) name is tagged with "MK:" in this output, and in this exam- ple it indicates which of the two alternatives matched. This is a more efficient way of obtaining this information than putting each alterna- tive in its own capturing parentheses.
If a verb with a name is encountered in a positive assertion that is true, the name is recorded and passed back if it is the last-encoun- tered. This does not happen for negative assertions or failing positive assertions.
After a partial match or a failed match, the last encountered name in the entire match process is returned. For example:
re> /X(*MARK:A)Y|X(*MARK:B)Z/mark data> XP No match, mark = B
Note that in this unanchored example the mark is retained from the match attempt that started at the letter "X" in the subject. Subsequent match attempts starting at "P" and then with an empty string do not get as far as the (*MARK) item, but nevertheless do not reset it.
If you are interested in (*MARK) values after failed matches, you should probably set the PCRE2_NO_START_OPTIMIZE option (see above) to ensure that the match is always attempted.
Verbs that act after backtracking
The following verbs do nothing when they are encountered. Matching con- tinues with what follows, but if there is a subsequent match failure, causing a backtrack to the verb, a failure is forced. That is, back- tracking cannot pass to the left of the verb. However, when one of these verbs appears inside an atomic group or in a lookaround assertion that is true, its effect is confined to that group, because once the group has been matched, there is never any backtracking into it. Back- tracking from beyond an assertion or an atomic group ignores the entire group, and seeks a preceding backtracking point.
These verbs differ in exactly what kind of failure occurs when back- tracking reaches them. The behaviour described below is what happens when the verb is not in a subroutine or an assertion. Subsequent sec- tions cover these special cases.
(*COMMIT) or (*COMMIT:NAME)
This verb causes the whole match to fail outright if there is a later matching failure that causes backtracking to reach it. Even if the pat- tern is unanchored, no further attempts to find a match by advancing the starting point take place. If (*COMMIT) is the only backtracking verb that is encountered, once it has been passed pcre2_match() is com- mitted to finding a match at the current starting point, or not at all. For example:
a+(*COMMIT)b
This matches "xxaab" but not "aacaab". It can be thought of as a kind of dynamic anchor, or "I've started, so I must finish."
The behaviour of (*COMMIT:NAME) is not the same as (*MARK:NAME)(*COM- MIT). It is like (*MARK:NAME) in that the name is remembered for pass- ing back to the caller. However, (*SKIP:NAME) searches only for names that are set with (*MARK), ignoring those set by any of the other back- tracking verbs.
If there is more than one backtracking verb in a pattern, a different one that follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a match does not always guarantee that a match must be at this starting point.
Note that (*COMMIT) at the start of a pattern is not the same as an an- chor, unless PCRE2's start-of-match optimizations are turned off, as shown in this output from pcre2test:
re> /(*COMMIT)abc/ data> xyzabc 0: abc data> re> /(*COMMIT)abc/no_start_optimize data> xyzabc No match
For the first pattern, PCRE2 knows that any match must start with "a", so the optimization skips along the subject to "a" before applying the pattern to the first set of data. The match attempt then succeeds. The second pattern disables the optimization that skips along to the first character. The pattern is now applied starting at "x", and so the (*COMMIT) causes the match to fail without trying any other starting points.
(*PRUNE) or (*PRUNE:NAME)
This verb causes the match to fail at the current starting position in the subject if there is a later matching failure that causes backtrack- ing to reach it. If the pattern is unanchored, the normal "bumpalong" advance to the next starting character then happens. Backtracking can occur as usual to the left of (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but if there is no match to the right, backtracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alternative to an atomic group or possessive quan- tifier, but there are some uses of (*PRUNE) that cannot be expressed in any other way. In an anchored pattern (*PRUNE) has the same effect as (*COMMIT).
The behaviour of (*PRUNE:NAME) is not the same as (*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in that the name is remembered for passing back to the caller. However, (*SKIP:NAME) searches only for names set with (*MARK), ignoring those set by other backtracking verbs.
(*SKIP)
This verb, when given without a name, is like (*PRUNE), except that if the pattern is unanchored, the "bumpalong" advance is not to the next character, but to the position in the subject where (*SKIP) was encoun- tered. (*SKIP) signifies that whatever text was matched leading up to it cannot be part of a successful match if there is a later mismatch. Consider:
a+(*SKIP)b
If the subject is "aaaac...", after the first match attempt fails (starting at the first character in the string), the starting point skips on to start the next attempt at "c". Note that a possessive quan- tifier does not have the same effect as this example; although it would suppress backtracking during the first match attempt, the second at- tempt would start at the second character instead of skipping on to "c".
If (*SKIP) is used to specify a new starting position that is the same as the starting position of the current match, or (by being inside a lookbehind) earlier, the position specified by (*SKIP) is ignored, and instead the normal "bumpalong" occurs.
(*SKIP:NAME)
When (*SKIP) has an associated name, its behaviour is modified. When such a (*SKIP) is triggered, the previous path through the pattern is searched for the most recent (*MARK) that has the same name. If one is found, the "bumpalong" advance is to the subject position that corre- sponds to that (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a matching name is found, the (*SKIP) is ignored.
The search for a (*MARK) name uses the normal backtracking mechanism, which means that it does not see (*MARK) settings that are inside atomic groups or assertions, because they are never re-entered by back- tracking. Compare the following pcre2test examples:
re> /a(?>(*MARK:X))(*SKIP:X)(*F)|(.)/ data: abc 0: a 1: a data: re> /a(?:(*MARK:X))(*SKIP:X)(*F)|(.)/ data: abc 0: b 1: b
In the first example, the (*MARK) setting is in an atomic group, so it is not seen when (*SKIP:X) triggers, causing the (*SKIP) to be ignored. This allows the second branch of the pattern to be tried at the first character position. In the second example, the (*MARK) setting is not in an atomic group. This allows (*SKIP:X) to find the (*MARK) when it backtracks, and this causes a new matching attempt to start at the sec- ond character. This time, the (*MARK) is never seen because "a" does not match "b", so the matcher immediately jumps to the second branch of the pattern.
Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores names that are set by other backtracking verbs.
(*THEN) or (*THEN:NAME)
This verb causes a skip to the next innermost alternative when back- tracking reaches it. That is, it cancels any further backtracking within the current alternative. Its name comes from the observation that it can be used for a pattern-based if-then-else block:
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
If the COND1 pattern matches, FOO is tried (and possibly further items after the end of the group if FOO succeeds); on failure, the matcher skips to the second alternative and tries COND2, without backtracking into COND1. If that succeeds and BAR fails, COND3 is tried. If subse- quently BAZ fails, there are no more alternatives, so there is a back- track to whatever came before the entire group. If (*THEN) is not in- side an alternation, it acts like (*PRUNE).
The behaviour of (*THEN:NAME) is not the same as (*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the name is remembered for passing back to the caller. However, (*SKIP:NAME) searches only for names set with (*MARK), ignoring those set by other backtracking verbs.
A group that does not contain a | character is just a part of the en- closing alternative; it is not a nested alternation with only one al- ternative. The effect of (*THEN) extends beyond such a group to the en- closing alternative. Consider this pattern, where A, B, etc. are com- plex pattern fragments that do not contain any | characters at this level:
A (B(*THEN)C) | D
If A and B are matched, but there is a failure in C, matching does not backtrack into A; instead it moves to the next alternative, that is, D. However, if the group containing (*THEN) is given an alternative, it behaves differently:
A (B(*THEN)C | (*FAIL)) | D
The effect of (*THEN) is now confined to the inner group. After a fail- ure in C, matching moves to (*FAIL), which causes the whole group to fail because there are no more alternatives to try. In this case, matching does backtrack into A.
Note that a conditional group is not considered as having two alterna- tives, because only one is ever used. In other words, the | character in a conditional group has a different meaning. Ignoring white space, consider:
^.*? (?(?=a) a | b(*THEN)c )
If the subject is "ba", this pattern does not match. Because .*? is un- greedy, it initially matches zero characters. The condition (?=a) then fails, the character "b" is matched, but "c" is not. At this point, matching does not backtrack to .*? as might perhaps be expected from the presence of the | character. The conditional group is part of the single alternative that comprises the whole pattern, and so the match fails. (If there was a backtrack into .*?, allowing it to match "b", the match would succeed.)
The verbs just described provide four different "strengths" of control when subsequent matching fails. (*THEN) is the weakest, carrying on the match at the next alternative. (*PRUNE) comes next, failing the match at the current starting position, but allowing an advance to the next character (for an unanchored pattern). (*SKIP) is similar, except that the advance may be more than one character. (*COMMIT) is the strongest, causing the entire match to fail.
More than one backtracking verb
If more than one backtracking verb is present in a pattern, the one that is backtracked onto first acts. For example, consider this pat- tern, where A, B, etc. are complex pattern fragments:
(A(*COMMIT)B(*THEN)C|ABD)
If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes the next alternative (ABD) to be tried. This behaviour is consistent, but is not always the same as Perl's. It means that if two or more backtracking verbs appear in succession, all but the last of them has no effect. Consider this example:
...(*COMMIT)(*PRUNE)...
If there is a matching failure to the right, backtracking onto (*PRUNE) causes it to be triggered, and its action is taken. There can never be a backtrack onto (*COMMIT).
Backtracking verbs in repeated groups
PCRE2 sometimes differs from Perl in its handling of backtracking verbs in repeated groups. For example, consider:
/(a(*COMMIT)b)+ac/
If the subject is "abac", Perl matches unless its optimizations are disabled, but PCRE2 always fails because the (*COMMIT) in the second repeat of the group acts.
Backtracking verbs in assertions
(*FAIL) in any assertion has its normal effect: it forces an immediate backtrack. The behaviour of the other backtracking verbs depends on whether or not the assertion is standalone or acting as the condition in a conditional group.
(*ACCEPT) in a standalone positive assertion causes the assertion to succeed without any further processing; captured strings and a mark name (if set) are retained. In a standalone negative assertion, (*AC- CEPT) causes the assertion to fail without any further processing; cap- tured substrings and any mark name are discarded.
If the assertion is a condition, (*ACCEPT) causes the condition to be true for a positive assertion and false for a negative one; captured substrings are retained in both cases.
The remaining verbs act only when a later failure causes a backtrack to reach them. This means that, for the Perl-compatible assertions, their effect is confined to the assertion, because Perl lookaround assertions are atomic. A backtrack that occurs after such an assertion is complete does not jump back into the assertion. Note in particular that a (*MARK) name that is set in an assertion is not "seen" by an instance of (*SKIP:NAME) later in the pattern.
PCRE2 now supports non-atomic positive assertions, as described in the section entitled "Non-atomic assertions" above. These assertions must be standalone (not used as conditions). They are not Perl-compatible. For these assertions, a later backtrack does jump back into the asser- tion, and therefore verbs such as (*COMMIT) can be triggered by back- tracks from later in the pattern.
The effect of (*THEN) is not allowed to escape beyond an assertion. If there are no more branches to try, (*THEN) causes a positive assertion to be false, and a negative assertion to be true.
The other backtracking verbs are not treated specially if they appear in a standalone positive assertion. In a conditional positive asser- tion, backtracking (from within the assertion) into (*COMMIT), (*SKIP), or (*PRUNE) causes the condition to be false. However, for both stand- alone and conditional negative assertions, backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes the assertion to be true, without consider- ing any further alternative branches.
Backtracking verbs in subroutines
These behaviours occur whether or not the group is called recursively.
(*ACCEPT) in a group called as a subroutine causes the subroutine match to succeed without any further processing. Matching then continues af- ter the subroutine call. Perl documents this behaviour. Perl's treat- ment of the other verbs in subroutines is different in some cases.
(*FAIL) in a group called as a subroutine has its normal effect: it forces an immediate backtrack.
(*COMMIT), (*SKIP), and (*PRUNE) cause the subroutine match to fail when triggered by being backtracked to in a group called as a subrou- tine. There is then a backtrack at the outer level.
(*THEN), when triggered, skips to the next alternative in the innermost enclosing group that has alternatives (its normal behaviour). However, if there is no such group within the subroutine's group, the subroutine match fails and there is a backtrack at the outer level.
SEE ALSO
pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2syntax(3), pcre2(3).
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
REVISION
Last updated: 04 June 2024 Copyright (c) 1997-2024 University of Cambridge.
PCRE2 10.44 04 June 2024 PCRE2PATTERN(3)------------------------------------------------------------------------------
PCRE2PERFORM(3) Library Functions Manual PCRE2PERFORM(3)
NAME PCRE2 - Perl-compatible regular expressions (revised API)
PCRE2 PERFORMANCE
Two aspects of performance are discussed below: memory usage and pro- cessing time. The way you express your pattern as a regular expression can affect both of them.
COMPILED PATTERN MEMORY USAGE
Patterns are compiled by PCRE2 into a reasonably efficient interpretive code, so that most simple patterns do not use much memory for storing the compiled version. However, there is one case where the memory usage of a compiled pattern can be unexpectedly large. If a parenthesized group has a quantifier with a minimum greater than 1 and/or a limited maximum, the whole group is repeated in the compiled code. For example, the pattern
(abc|def){2,4}
is compiled as if it were
(abc|def)(abc|def)((abc|def)(abc|def)?)?
(Technical aside: It is done this way so that backtrack points within each of the repetitions can be independently maintained.)
For regular expressions whose quantifiers use only small numbers, this is not usually a problem. However, if the numbers are large, and par- ticularly if such repetitions are nested, the memory usage can become an embarrassment. For example, the very simple pattern
((ab){1,1000}c){1,3}
uses over 50KiB when compiled using the 8-bit library. When PCRE2 is compiled with its default internal pointer size of two bytes, the size limit on a compiled pattern is 65535 code units in the 8-bit and 16-bit libraries, and this is reached with the above pattern if the outer rep- etition is increased from 3 to 4. PCRE2 can be compiled to use larger internal pointers and thus handle larger compiled patterns, but it is better to try to rewrite your pattern to use less memory if you can.
One way of reducing the memory usage for such patterns is to make use of PCRE2's "subroutine" facility. Re-writing the above pattern as
((ab)(?2){0,999}c)(?1){0,2}
reduces the memory requirements to around 16KiB, and indeed it remains under 20KiB even with the outer repetition increased to 100. However, this kind of pattern is not always exactly equivalent, because any cap- tures within subroutine calls are lost when the subroutine completes. If this is not a problem, this kind of rewriting will allow you to process patterns that PCRE2 cannot otherwise handle. The matching per- formance of the two different versions of the pattern are roughly the same. (This applies from release 10.30 - things were different in ear- lier releases.)
STACK AND HEAP USAGE AT RUN TIME
From release 10.30, the interpretive (non-JIT) version of pcre2_match() uses very little system stack at run time. In earlier releases recur- sive function calls could use a great deal of stack, and this could cause problems, but this usage has been eliminated. Backtracking posi- tions are now explicitly remembered in memory frames controlled by the code.
The size of each frame depends on the size of pointer variables and the number of capturing parenthesized groups in the pattern being matched. On a 64-bit system the frame size for a pattern with no captures is 128 bytes. For each capturing group the size increases by 16 bytes.
Until release 10.41, an initial 20KiB frames vector was allocated on the system stack, but this still caused some issues for multi-thread applications where each thread has a very small stack. From release 10.41 backtracking memory frames are always held in heap memory. An initial heap allocation is obtained the first time any match data block is passed to pcre2_match(). This is remembered with the match data block and re-used if that block is used for another match. It is freed when the match data block itself is freed.
The size of the initial block is the larger of 20KiB or ten times the pattern's frame size, unless the heap limit is less than this, in which case the heap limit is used. If the initial block proves to be too small during matching, it is replaced by a larger block, subject to the heap limit. The heap limit is checked only when a new block is to be allocated. Reducing the heap limit between calls to pcre2_match() with the same match data block does not affect the saved block.
In contrast to pcre2_match(), pcre2_dfa_match() does use recursive function calls, but only for processing atomic groups, lookaround as- sertions, and recursion within the pattern. The original version of the code used to allocate quite large internal workspace vectors on the stack, which caused some problems for some patterns in environments with small stacks. From release 10.32 the code for pcre2_dfa_match() has been re-factored to use heap memory when necessary for internal workspace when recursing, though recursive function calls are still used.
The "match depth" parameter can be used to limit the depth of function recursion, and the "match heap" parameter to limit heap memory in pcre2_dfa_match().
PROCESSING TIME
Certain items in regular expression patterns are processed more effi- ciently than others. It is more efficient to use a character class like [aeiou] than a set of single-character alternatives such as (a|e|i|o|u). In general, the simplest construction that provides the required behaviour is usually the most efficient. Jeffrey Friedl's book contains a lot of useful general discussion about optimizing regular expressions for efficient performance. This document contains a few ob- servations about PCRE2.
Using Unicode character properties (the \p, \P, and \X escapes) is slow, because PCRE2 has to use a multi-stage table lookup whenever it needs a character's property. If you can find an alternative pattern that does not use character properties, it will probably be faster.
By default, the escape sequences \b, \d, \s, and \w, and the POSIX character classes such as [:alpha:] do not use Unicode properties, partly for backwards compatibility, and partly for performance reasons. However, you can set the PCRE2_UCP option or start the pattern with (*UCP) if you want Unicode character properties to be used. This can double the matching time for items such as \d, when matched with pcre2_match(); the performance loss is less with a DFA matching func- tion, and in both cases there is not much difference for \b.
When a pattern begins with .* not in atomic parentheses, nor in paren- theses that are the subject of a backreference, and the PCRE2_DOTALL option is set, the pattern is implicitly anchored by PCRE2, since it can match only at the start of a subject string. If the pattern has multiple top-level branches, they must all be anchorable. The optimiza- tion can be disabled by the PCRE2_NO_DOTSTAR_ANCHOR option, and is au- tomatically disabled if the pattern contains (*PRUNE) or (*SKIP).
If PCRE2_DOTALL is not set, PCRE2 cannot make this optimization, be- cause the dot metacharacter does not then match a newline, and if the subject string contains newlines, the pattern may match from the char- acter immediately following one of them instead of from the very start. For example, the pattern
.*second
matches the subject "first\nand second" (where \n stands for a newline character), with the match starting at the seventh character. In order to do this, PCRE2 has to retry the match starting after every newline in the subject.
If you are using such a pattern with subject strings that do not con- tain newlines, the best performance is obtained by setting PCRE2_DOTALL, or starting the pattern with ^.* or ^.*? to indicate ex- plicit anchoring. That saves PCRE2 from having to scan along the sub- ject looking for a newline to restart at.
Beware of patterns that contain nested indefinite repeats. These can take a long time to run when applied to a string that does not match. Consider the pattern fragment
^(a+)*
This can match "aaaa" in 16 different ways, and this number increases very rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4 times, and for each of those cases other than 0 or 4, the + repeats can match different numbers of times.) When the remainder of the pattern is such that the entire match is going to fail, PCRE2 has in principle to try every possible variation, and this can take an ex- tremely long time, even for relatively short strings.
An optimization catches some of the more simple cases such as
(a+)*b
where a literal character follows. Before embarking on the standard matching procedure, PCRE2 checks that there is a "b" later in the sub- ject string, and if there is not, it fails the match immediately. How- ever, when there is no following literal this optimization cannot be used. You can see the difference by comparing the behaviour of
(a+)*\d
with the pattern above. The former gives a failure almost instantly when applied to a whole line of "a" characters, whereas the latter takes an appreciable time with strings longer than about 20 characters.
In many cases, the solution to this kind of performance issue is to use an atomic group or a possessive quantifier. This can often reduce mem- ory requirements as well. As another example, consider this pattern:
([^<]|<(?!inet))+
It matches from wherever it starts until it encounters "<inet" or the end of the data, and is the kind of pattern that might be used when processing an XML file. Each iteration of the outer parentheses matches either one character that is not "<" or a "<" that is not followed by "inet". However, each time a parenthesis is processed, a backtracking position is passed, so this formulation uses a memory frame for each matched character. For a long string, a lot of memory is required. Con- sider now this rewritten pattern, which matches exactly the same strings:
([^<]++|<(?!inet))+
This runs much faster, because sequences of characters that do not con- tain "<" are "swallowed" in one item inside the parentheses, and a pos- sessive quantifier is used to stop any backtracking into the runs of non-"<" characters. This version also uses a lot less memory because entry to a new set of parentheses happens only when a "<" character that is not followed by "inet" is encountered (and we assume this is relatively rare).
This example shows that one way of optimizing performance when matching long subject strings is to write repeated parenthesized subpatterns to match more than one character whenever possible.
SETTING RESOURCE LIMITS
You can set limits on the amount of processing that takes place when matching, and on the amount of heap memory that is used. The default values of the limits are very large, and unlikely ever to operate. They can be changed when PCRE2 is built, and they can also be set when pcre2_match() or pcre2_dfa_match() is called. For details of these in- terfaces, see the pcre2build documentation and the section entitled "The match context" in the pcre2api documentation.
The pcre2test test program has a modifier called "find_limits" which, if applied to a subject line, causes it to find the smallest limits that allow a pattern to match. This is done by repeatedly matching with different limits.
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
REVISION
Last updated: 27 July 2022 Copyright (c) 1997-2022 University of Cambridge.
PCRE2 10.41 27 July 2022 PCRE2PERFORM(3)------------------------------------------------------------------------------
PCRE2POSIX(3) Library Functions Manual PCRE2POSIX(3)
NAME PCRE2 - Perl-compatible regular expressions (revised API)
SYNOPSIS
#include <pcre2posix.h>
int pcre2_regcomp(regex_t *preg, const char *pattern, int cflags);
int pcre2_regexec(const regex_t *preg, const char *string, size_t nmatch, regmatch_t pmatch[], int eflags);
size_t pcre2_regerror(int errcode, const regex_t *preg, char *errbuf, size_t errbuf_size);
void pcre2_regfree(regex_t *preg);
DESCRIPTION
This set of functions provides a POSIX-style API for the PCRE2 regular expression 8-bit library. There are no POSIX-style wrappers for PCRE2's 16-bit and 32-bit libraries. See the pcre2api documentation for a de- scription of PCRE2's native API, which contains much additional func- tionality.
IMPORTANT NOTE: The functions described here are NOT thread-safe, and should not be used in multi-threaded applications. They are also lim- ited to processing subjects that are not bigger than 2GB. Use the na- tive API instead.
These functions are wrapper functions that ultimately call the PCRE2 native API. Their prototypes are defined in the pcre2posix.h header file, and they all have unique names starting with pcre2_. However, the pcre2posix.h header also contains macro definitions that convert the standard POSIX names such regcomp() into pcre2_regcomp() etc. This means that a program can use the usual POSIX names without running the risk of accidentally linking with POSIX functions from a different li- brary.
On Unix-like systems the PCRE2 POSIX library is called libpcre2-posix, so can be accessed by adding -lpcre2-posix to the command for linking an application. Because the POSIX functions call the native ones, it is also necessary to add -lpcre2-8.
On Windows systems, if you are linking to a DLL version of the library, it is recommended that PCRE2POSIX_SHARED is defined before including the pcre2posix.h header, as it will allow for a more efficient way to invoke the functions by adding the __declspec(dllimport) decorator.
Although they were not defined as prototypes in pcre2posix.h, releases 10.33 to 10.36 of the library contained functions with the POSIX names regcomp() etc. These simply passed their arguments to the PCRE2 func- tions. These functions were provided for backwards compatibility with earlier versions of PCRE2, which had only POSIX names. However, this has proved troublesome in situations where a program links with several libraries, some of which use PCRE2's POSIX interface while others use the real POSIX functions. For this reason, the POSIX names have been removed since release 10.37.
Calling the header file pcre2posix.h avoids any conflict with other POSIX libraries. It can, of course, be renamed or aliased as regex.h, which is the "correct" name, if there is no clash. It provides two structure types, regex_t for compiled internal forms, and regmatch_t for returning captured substrings. It also defines some constants whose names start with "REG_"; these are used for setting options and identi- fying error codes.
USING THE POSIX FUNCTIONS
Note that these functions are just POSIX-style wrappers for PCRE2's na- tive API. They do not give POSIX regular expression behaviour, and they are not thread-safe or even POSIX compatible.
Those POSIX option bits that can reasonably be mapped to PCRE2 native options have been implemented. In addition, the option REG_EXTENDED is defined with the value zero. This has no effect, but since programs that are written to the POSIX interface often use it, this makes it easier to slot in PCRE2 as a replacement library. Other POSIX options are not even defined.
There are also some options that are not defined by POSIX. These have been added at the request of users who want to make use of certain PCRE2-specific features via the POSIX calling interface or to add BSD or GNU functionality.
When PCRE2 is called via these functions, it is only the API that is POSIX-like in style. The syntax and semantics of the regular expres- sions themselves are still those of Perl, subject to the setting of various PCRE2 options, as described below. "POSIX-like in style" means that the API approximates to the POSIX definition; it is not fully POSIX-compatible, and in multi-unit encoding domains it is probably even less compatible.
The descriptions below use the actual names of the functions, but, as described above, the standard POSIX names (without the pcre2_ prefix) may also be used.
COMPILING A PATTERN
The function pcre2_regcomp() is called to compile a pattern into an in- ternal form. By default, the pattern is a C string terminated by a bi- nary zero (but see REG_PEND below). The preg argument is a pointer to a regex_t structure that is used as a base for storing information about the compiled regular expression. It is also used for input when REG_PEND is set. The regex_t structure used by pcre2_regcomp() is de- fined in pcre2posix.h and is not the same as the structure used by other libraries that provide POSIX-style matching.
The argument cflags is either zero, or contains one or more of the bits defined by the following macros:
REG_DOTALL
The PCRE2_DOTALL option is set when the regular expression is passed for compilation to the native function. Note that REG_DOTALL is not part of the POSIX standard.
REG_ICASE
The PCRE2_CASELESS option is set when the regular expression is passed for compilation to the native function.
REG_NEWLINE
The PCRE2_MULTILINE option is set when the regular expression is passed for compilation to the native function. Note that this does not mimic the defined POSIX behaviour for REG_NEWLINE (see the following sec- tion).
REG_NOSPEC
The PCRE2_LITERAL option is set when the regular expression is passed for compilation to the native function. This disables all meta charac- ters in the pattern, causing it to be treated as a literal string. The only other options that are allowed with REG_NOSPEC are REG_ICASE, REG_NOSUB, REG_PEND, and REG_UTF. Note that REG_NOSPEC is not part of the POSIX standard.
REG_NOSUB
When a pattern that is compiled with this flag is passed to pcre2_regexec() for matching, the nmatch and pmatch arguments are ig- nored, and no captured strings are returned. Versions of the PCRE li- brary prior to 10.22 used to set the PCRE2_NO_AUTO_CAPTURE compile op- tion, but this no longer happens because it disables the use of back- references.
REG_PEND
If this option is set, the reg_endp field in the preg structure (which has the type const char *) must be set to point to the character beyond the end of the pattern before calling pcre2_regcomp(). The pattern it- self may now contain binary zeros, which are treated as data charac- ters. Without REG_PEND, a binary zero terminates the pattern and the re_endp field is ignored. This is a GNU extension to the POSIX standard and should be used with caution in software intended to be portable to other systems.
REG_UCP
The PCRE2_UCP option is set when the regular expression is passed for compilation to the native function. This causes PCRE2 to use Unicode properties when matching \d, \w, etc., instead of just recognizing ASCII values. Note that REG_UCP is not part of the POSIX standard.
REG_UNGREEDY
The PCRE2_UNGREEDY option is set when the regular expression is passed for compilation to the native function. Note that REG_UNGREEDY is not part of the POSIX standard.
REG_UTF
The PCRE2_UTF option is set when the regular expression is passed for compilation to the native function. This causes the pattern itself and all data strings used for matching it to be treated as UTF-8 strings. Note that REG_UTF is not part of the POSIX standard.
In the absence of these flags, no options are passed to the native function. This means that the regex is compiled with PCRE2 default se- mantics. In particular, the way it handles newline characters in the subject string is the Perl way, not the POSIX way. Note that setting PCRE2_MULTILINE has only some of the effects specified for REG_NEWLINE. It does not affect the way newlines are matched by the dot metacharac- ter (they are not) or by a negative class such as [^a] (they are).
The yield of pcre2_regcomp() is zero on success, and non-zero other- wise. The preg structure is filled in on success, and one other member of the structure (as well as re_endp) is public: re_nsub contains the number of capturing subpatterns in the regular expression. Various er- ror codes are defined in the header file.
NOTE: If the yield of pcre2_regcomp() is non-zero, you must not attempt to use the contents of the preg structure. If, for example, you pass it to pcre2_regexec(), the result is undefined and your program is likely to crash.
MATCHING NEWLINE CHARACTERS
This area is not simple, because POSIX and Perl take different views of things. It is not possible to get PCRE2 to obey POSIX semantics, but then PCRE2 was never intended to be a POSIX engine. The following table lists the different possibilities for matching newline characters in Perl and PCRE2:
Default Change with
. matches newline no PCRE2_DOTALL newline matches [^a] yes not changeable $ matches \n at end yes PCRE2_DOLLAR_ENDONLY $ matches \n in middle no PCRE2_MULTILINE ^ matches \n in middle no PCRE2_MULTILINE
This is the equivalent table for a POSIX-compatible pattern matcher:
Default Change with
. matches newline yes REG_NEWLINE newline matches [^a] yes REG_NEWLINE $ matches \n at end no REG_NEWLINE $ matches \n in middle no REG_NEWLINE ^ matches \n in middle no REG_NEWLINE
This behaviour is not what happens when PCRE2 is called via its POSIX API. By default, PCRE2's behaviour is the same as Perl's, except that there is no equivalent for PCRE2_DOLLAR_ENDONLY in Perl. In both PCRE2 and Perl, there is no way to stop newline from matching [^a].
Default POSIX newline handling can be obtained by setting PCRE2_DOTALL and PCRE2_DOLLAR_ENDONLY when calling pcre2_compile() directly, but there is no way to make PCRE2 behave exactly as for the REG_NEWLINE ac- tion. When using the POSIX API, passing REG_NEWLINE to PCRE2's pcre2_regcomp() function causes PCRE2_MULTILINE to be passed to pcre2_compile(), and REG_DOTALL passes PCRE2_DOTALL. There is no way to pass PCRE2_DOLLAR_ENDONLY.
MATCHING A PATTERN
The function pcre2_regexec() is called to match a compiled pattern preg against a given string, which is by default terminated by a zero byte (but see REG_STARTEND below), subject to the options in eflags. These can be:
REG_NOTBOL
The PCRE2_NOTBOL option is set when calling the underlying PCRE2 match- ing function.
REG_NOTEMPTY
The PCRE2_NOTEMPTY option is set when calling the underlying PCRE2 matching function. Note that REG_NOTEMPTY is not part of the POSIX standard. However, setting this option can give more POSIX-like behav- iour in some situations.
REG_NOTEOL
The PCRE2_NOTEOL option is set when calling the underlying PCRE2 match- ing function.
REG_STARTEND
When this option is set, the subject string starts at string + pmatch[0].rm_so and ends at string + pmatch[0].rm_eo, which should point to the first character beyond the string. There may be binary ze- ros within the subject string, and indeed, using REG_STARTEND is the only way to pass a subject string that contains a binary zero.
Whatever the value of pmatch[0].rm_so, the offsets of the matched string and any captured substrings are still given relative to the start of string itself. (Before PCRE2 release 10.30 these were given relative to string + pmatch[0].rm_so, but this differs from other im- plementations.)
This is a BSD extension, compatible with but not specified by IEEE Standard 1003.2 (POSIX.2), and should be used with caution in software intended to be portable to other systems. Note that a non-zero rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location and length of the string, not how it is matched. Setting REG_STARTEND and passing pmatch as NULL are mutually exclusive; the error REG_INVARG is returned.
If the pattern was compiled with the REG_NOSUB flag, no data about any matched strings is returned. The nmatch and pmatch arguments of pcre2_regexec() are ignored (except possibly as input for REG_STAR- TEND).
The value of nmatch may be zero, and the value pmatch may be NULL (un- less REG_STARTEND is set); in both these cases no data about any matched strings is returned.
Otherwise, the portion of the string that was matched, and also any captured substrings, are returned via the pmatch argument, which points to an array of nmatch structures of type regmatch_t, containing the members rm_so and rm_eo. These contain the byte offset to the first character of each substring and the offset to the first character after the end of each substring, respectively. The 0th element of the vector relates to the entire portion of string that was matched; subsequent elements relate to the capturing subpatterns of the regular expression. Unused entries in the array have both structure members set to -1.
regmatch_t as well as the regoff_t typedef it uses are defined in pcre2posix.h and are not warranted to have the same size or layout as other similarly named types from other libraries that provide POSIX- style matching.
A successful match yields a zero return; various error codes are de- fined in the header file, of which REG_NOMATCH is the "expected" fail- ure code.
ERROR MESSAGES
The pcre2_regerror() function maps a non-zero errorcode from either pcre2_regcomp() or pcre2_regexec() to a printable message. If preg is not NULL, the error should have arisen from the use of that structure. A message terminated by a binary zero is placed in errbuf. If the buffer is too short, only the first errbuf_size - 1 characters of the error message are used. The yield of the function is the size of buffer needed to hold the whole message, including the terminating zero. This value is greater than errbuf_size if the message was truncated.
MEMORY USAGE
Compiling a regular expression causes memory to be allocated and asso- ciated with the preg structure. The function pcre2_regfree() frees all such memory, after which preg may no longer be used as a compiled ex- pression.
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
REVISION
Last updated: 19 January 2024 Copyright (c) 1997-2024 University of Cambridge.
PCRE2 10.43 19 January 2024 PCRE2POSIX(3)------------------------------------------------------------------------------
PCRE2SAMPLE(3) Library Functions Manual PCRE2SAMPLE(3)
NAME PCRE2 - Perl-compatible regular expressions (revised API)
PCRE2 SAMPLE PROGRAM
A simple, complete demonstration program to get you started with using PCRE2 is supplied in the file pcre2demo.c in the src directory in the PCRE2 distribution. A listing of this program is given in the pcre2demo documentation. If you do not have a copy of the PCRE2 distribution, you can save this listing to re-create the contents of pcre2demo.c.
The demonstration program compiles the regular expression that is its first argument, and matches it against the subject string in its second argument. No PCRE2 options are set, and default character tables are used. If matching succeeds, the program outputs the portion of the sub- ject that matched, together with the contents of any captured sub- strings.
If the -g option is given on the command line, the program then goes on to check for further matches of the same regular expression in the same subject string. The logic is a little bit tricky because of the possi- bility of matching an empty string. Comments in the code explain what is going on.
The code in pcre2demo.c is an 8-bit program that uses the PCRE2 8-bit library. It handles strings and characters that are stored in 8-bit code units. By default, one character corresponds to one code unit, but if the pattern starts with "(*UTF)", both it and the subject are treated as UTF-8 strings, where characters may occupy multiple code units.
If PCRE2 is installed in the standard include and library directories for your operating system, you should be able to compile the demonstra- tion program using a command like this:
cc -o pcre2demo pcre2demo.c -lpcre2-8
If PCRE2 is installed elsewhere, you may need to add additional options to the command line. For example, on a Unix-like system that has PCRE2 installed in /usr/local, you can compile the demonstration program us- ing a command like this:
cc -o pcre2demo -I/usr/local/include pcre2demo.c \ -L/usr/local/lib -lpcre2-8
Once you have built the demonstration program, you can run simple tests like this:
./pcre2demo 'cat|dog' 'the cat sat on the mat' ./pcre2demo -g 'cat|dog' 'the dog sat on the cat'
Note that there is a much more comprehensive test program, called pcre2test, which supports many more facilities for testing regular ex- pressions using all three PCRE2 libraries (8-bit, 16-bit, and 32-bit, though not all three need be installed). The pcre2demo program is pro- vided as a relatively simple coding example.
If you try to run pcre2demo when PCRE2 is not installed in the standard library directory, you may get an error like this on some operating systems (e.g. Solaris):
ld.so.1: pcre2demo: fatal: libpcre2-8.so.0: open failed: No such file or directory
This is caused by the way shared library support works on those sys- tems. You need to add
-R/usr/local/lib
(for example) to the compile command to get round this problem.
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
REVISION
Last updated: 02 February 2016 Copyright (c) 1997-2016 University of Cambridge.
PCRE2 10.22 02 February 2016 PCRE2SAMPLE(3)------------------------------------------------------------------------------
PCRE2SERIALIZE(3) Library Functions Manual PCRE2SERIALIZE(3)
NAME PCRE2 - Perl-compatible regular expressions (revised API)
SAVING AND RE-USING PRECOMPILED PCRE2 PATTERNS
int32_t pcre2_serialize_decode(pcre2_code **codes, int32_t number_of_codes, const uint8_t *bytes, pcre2_general_context *gcontext);
int32_t pcre2_serialize_encode(const pcre2_code **codes, int32_t number_of_codes, uint8_t **serialized_bytes, PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext);
void pcre2_serialize_free(uint8_t *bytes);
int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes);
If you are running an application that uses a large number of regular expression patterns, it may be useful to store them in a precompiled form instead of having to compile them every time the application is run. However, if you are using the just-in-time optimization feature, it is not possible to save and reload the JIT data, because it is posi- tion-dependent. The host on which the patterns are reloaded must be running the same version of PCRE2, with the same code unit width, and must also have the same endianness, pointer width and PCRE2_SIZE type. For example, patterns compiled on a 32-bit system using PCRE2's 16-bit library cannot be reloaded on a 64-bit system, nor can they be reloaded using the 8-bit library.
Note that "serialization" in PCRE2 does not convert compiled patterns to an abstract format like Java or .NET serialization. The serialized output is really just a bytecode dump, which is why it can only be re- loaded in the same environment as the one that created it. Hence the restrictions mentioned above. Applications that are not statically linked with a fixed version of PCRE2 must be prepared to recompile pat- terns from their sources, in order to be immune to PCRE2 upgrades.
SECURITY CONCERNS
The facility for saving and restoring compiled patterns is intended for use within individual applications. As such, the data supplied to pcre2_serialize_decode() is expected to be trusted data, not data from arbitrary external sources. There is only some simple consistency checking, not complete validation of what is being re-loaded. Corrupted data may cause undefined results. For example, if the length field of a pattern in the serialized data is corrupted, the deserializing code may read beyond the end of the byte stream that is passed to it.
SAVING COMPILED PATTERNS
Before compiled patterns can be saved they must be serialized, which in PCRE2 means converting the pattern to a stream of bytes. A single byte stream may contain any number of compiled patterns, but they must all use the same character tables. A single copy of the tables is included in the byte stream (its size is 1088 bytes). For more details of char- acter tables, see the section on locale support in the pcre2api docu- mentation.
The function pcre2_serialize_encode() creates a serialized byte stream from a list of compiled patterns. Its first two arguments specify the list, being a pointer to a vector of pointers to compiled patterns, and the length of the vector. The third and fourth arguments point to vari- ables which are set to point to the created byte stream and its length, respectively. The final argument is a pointer to a general context, which can be used to specify custom memory management functions. If this argument is NULL, malloc() is used to obtain memory for the byte stream. The yield of the function is the number of serialized patterns, or one of the following negative error codes:
PCRE2_ERROR_BADDATA the number of patterns is zero or less PCRE2_ERROR_BADMAGIC mismatch of id bytes in one of the patterns PCRE2_ERROR_NOMEMORY memory allocation failed PCRE2_ERROR_MIXEDTABLES the patterns do not all use the same tables PCRE2_ERROR_NULL the 1st, 3rd, or 4th argument is NULL
PCRE2_ERROR_BADMAGIC means either that a pattern's code has been cor- rupted, or that a slot in the vector does not point to a compiled pat- tern.
Once a set of patterns has been serialized you can save the data in any appropriate manner. Here is sample code that compiles two patterns and writes them to a file. It assumes that the variable fd refers to a file that is open for output. The error checking that should be present in a real application has been omitted for simplicity.
int errorcode; uint8_t *bytes; PCRE2_SIZE erroroffset; PCRE2_SIZE bytescount; pcre2_code *list_of_codes[2]; list_of_codes[0] = pcre2_compile("first pattern", PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL); list_of_codes[1] = pcre2_compile("second pattern", PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL); errorcode = pcre2_serialize_encode(list_of_codes, 2, &bytes, &bytescount, NULL); errorcode = fwrite(bytes, 1, bytescount, fd);
Note that the serialized data is binary data that may contain any of the 256 possible byte values. On systems that make a distinction be- tween binary and non-binary data, be sure that the file is opened for binary output.
Serializing a set of patterns leaves the original data untouched, so they can still be used for matching. Their memory must eventually be freed in the usual way by calling pcre2_code_free(). When you have fin- ished with the byte stream, it too must be freed by calling pcre2_seri- alize_free(). If this function is called with a NULL argument, it re- turns immediately without doing anything.
RE-USING PRECOMPILED PATTERNS
In order to re-use a set of saved patterns you must first make the se- rialized byte stream available in main memory (for example, by reading from a file). The management of this memory block is up to the applica- tion. You can use the pcre2_serialize_get_number_of_codes() function to find out how many compiled patterns are in the serialized data without actually decoding the patterns:
uint8_t *bytes = <serialized data>; int32_t number_of_codes = pcre2_serialize_get_number_of_codes(bytes);
The pcre2_serialize_decode() function reads a byte stream and recreates the compiled patterns in new memory blocks, setting pointers to them in a vector. The first two arguments are a pointer to a suitable vector and its length, and the third argument points to a byte stream. The fi- nal argument is a pointer to a general context, which can be used to specify custom memory management functions for the decoded patterns. If this argument is NULL, malloc() and free() are used. After deserializa- tion, the byte stream is no longer needed and can be discarded.
pcre2_code *list_of_codes[2]; uint8_t *bytes = <serialized data>; int32_t number_of_codes = pcre2_serialize_decode(list_of_codes, 2, bytes, NULL);
If the vector is not large enough for all the patterns in the byte stream, it is filled with those that fit, and the remainder are ig- nored. The yield of the function is the number of decoded patterns, or one of the following negative error codes:
PCRE2_ERROR_BADDATA second argument is zero or less PCRE2_ERROR_BADMAGIC mismatch of id bytes in the data PCRE2_ERROR_BADMODE mismatch of code unit size or PCRE2 version PCRE2_ERROR_BADSERIALIZEDDATA other sanity check failure PCRE2_ERROR_MEMORY memory allocation failed PCRE2_ERROR_NULL first or third argument is NULL
PCRE2_ERROR_BADMAGIC may mean that the data is corrupt, or that it was compiled on a system with different endianness.
Decoded patterns can be used for matching in the usual way, and must be freed by calling pcre2_code_free(). However, be aware that there is a potential race issue if you are using multiple patterns that were de- coded from a single byte stream in a multithreaded application. A sin- gle copy of the character tables is used by all the decoded patterns and a reference count is used to arrange for its memory to be automati- cally freed when the last pattern is freed, but there is no locking on this reference count. Therefore, if you want to call pcre2_code_free() for these patterns in different threads, you must arrange your own locking, and ensure that pcre2_code_free() cannot be called by two threads at the same time.
If a pattern was processed by pcre2_jit_compile() before being serial- ized, the JIT data is discarded and so is no longer available after a save/restore cycle. You can, however, process a restored pattern with pcre2_jit_compile() if you wish.
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
REVISION
Last updated: 27 June 2018 Copyright (c) 1997-2018 University of Cambridge.
PCRE2 10.32 27 June 2018 PCRE2SERIALIZE(3)------------------------------------------------------------------------------
PCRE2SYNTAX(3) Library Functions Manual PCRE2SYNTAX(3)
NAME PCRE2 - Perl-compatible regular expressions (revised API)
PCRE2 REGULAR EXPRESSION SYNTAX SUMMARY
The full syntax and semantics of the regular expressions that are sup- ported by PCRE2 are described in the pcre2pattern documentation. This document contains a quick-reference summary of the syntax.
QUOTING
\x where x is non-alphanumeric is a literal x \Q...\E treat enclosed characters as literal
Note that white space inside \Q...\E is always treated as literal, even if PCRE2_EXTENDED is set, causing most other white space to be ignored.
BRACED ITEMS
With one exception, wherever brace characters { and } are required to enclose data for constructions such as \g{2} or \k{name}, space and/or horizontal tab characters that follow { or precede } are allowed and are ignored. In the case of quantifiers, they may also appear before or after the comma. The exception is \u{...} which is not Perl-compatible and is recognized only when PCRE2_EXTRA_ALT_BSUX is set. This is an EC- MAScript compatibility feature, and follows ECMAScript's behaviour.
ESCAPED CHARACTERS
This table applies to ASCII and Unicode environments. An unrecognized escape sequence causes an error.
\a alarm, that is, the BEL character (hex 07) \cx "control-x", where x is a non-control ASCII character \e escape (hex 1B) \f form feed (hex 0C) \n newline (hex 0A) \r carriage return (hex 0D) \t tab (hex 09) \0dd character with octal code 0dd \ddd character with octal code ddd, or backreference \o{ddd..} character with octal code ddd.. \N{U+hh..} character with Unicode code point hh.. (Unicode mode only) \xhh character with hex code hh \x{hh..} character with hex code hh..
If PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX is set ("ALT_BSUX mode"), the following are also recognized:
\U the character "U" \uhhhh character with hex code hhhh \u{hh..} character with hex code hh.. but only for EXTRA_ALT_BSUX
When \x is not followed by {, from zero to two hexadecimal digits are read, but in ALT_BSUX mode \x must be followed by two hexadecimal dig- its to be recognized as a hexadecimal escape; otherwise it matches a literal "x". Likewise, if \u (in ALT_BSUX mode) is not followed by four hexadecimal digits or (in EXTRA_ALT_BSUX mode) a sequence of hex digits in curly brackets, it matches a literal "u".
Note that \0dd is always an octal code. The treatment of backslash fol- lowed by a non-zero digit is complicated; for details see the section "Non-printing characters" in the pcre2pattern documentation, where de- tails of escape processing in EBCDIC environments are also given. \N{U+hh..} is synonymous with \x{hh..} in PCRE2 but is not supported in EBCDIC environments. Note that \N not followed by an opening curly bracket has a different meaning (see below).
CHARACTER TYPES
. any character except newline; in dotall mode, any character whatsoever \C one code unit, even in UTF mode (best avoided) \d a decimal digit \D a character that is not a decimal digit \h a horizontal white space character \H a character that is not a horizontal white space character \N a character that is not a newline \p{xx} a character with the xx property \P{xx} a character without the xx property \R a newline sequence \s a white space character \S a character that is not a white space character \v a vertical white space character \V a character that is not a vertical white space character \w a "word" character \W a "non-word" character \X a Unicode extended grapheme cluster
\C is dangerous because it may leave the current matching point in the middle of a UTF-8 or UTF-16 character. The application can lock out the use of \C by setting the PCRE2_NEVER_BACKSLASH_C option. It is also possible to build PCRE2 with the use of \C permanently disabled.
By default, \d, \s, and \w match only ASCII characters, even in UTF-8 mode or in the 16-bit and 32-bit libraries. However, if locale-specific matching is happening, \s and \w may also match characters with code points in the range 128-255. If the PCRE2_UCP option is set, the behav- iour of these escape sequences is changed to use Unicode properties and they match many more characters, but there are some option settings that can restrict individual sequences to matching only ASCII charac- ters.
Property descriptions in \p and \P are matched caselessly; hyphens, un- derscores, and white space are ignored, in accordance with Unicode's "loose matching" rules.
GENERAL CATEGORY PROPERTIES FOR \p and \P
C Other Cc Control Cf Format Cn Unassigned Co Private use Cs Surrogate
L Letter Ll Lower case letter Lm Modifier letter Lo Other letter Lt Title case letter Lu Upper case letter Lc Ll, Lu, or Lt L& Ll, Lu, or Lt
M Mark Mc Spacing mark Me Enclosing mark Mn Non-spacing mark
N Number Nd Decimal number Nl Letter number No Other number
P Punctuation Pc Connector punctuation Pd Dash punctuation Pe Close punctuation Pf Final punctuation Pi Initial punctuation Po Other punctuation Ps Open punctuation
S Symbol Sc Currency symbol Sk Modifier symbol Sm Mathematical symbol So Other symbol
Z Separator Zl Line separator Zp Paragraph separator Zs Space separator
PCRE2 SPECIAL CATEGORY PROPERTIES FOR \p and \P
Xan Alphanumeric: union of properties L and N Xps POSIX space: property Z or tab, NL, VT, FF, CR Xsp Perl space: property Z or tab, NL, VT, FF, CR Xuc Universally-named character: one that can be represented by a Universal Character Name Xwd Perl word: property Xan or underscore
Perl and POSIX space are now the same. Perl added VT to its space char- acter set at release 5.18.
BINARY PROPERTIES FOR \p AND \P
Unicode defines a number of binary properties, that is, properties whose only values are true or false. You can obtain a list of those that are recognized by \p and \P, along with their abbreviations, by running this command:
pcre2test -LP
SCRIPT MATCHING WITH \p AND \P
Many script names and their 4-letter abbreviations are recognized in \p{sc:...} or \p{scx:...} items, or on their own with \p (and also \P of course). You can obtain a list of these scripts by running this com- mand:
pcre2test -LS
THE BIDI_CLASS PROPERTY FOR \p AND \P
\p{Bidi_Class:<class>} matches a character with the given class \p{BC:<class>} matches a character with the given class
The recognized classes are:
AL Arabic letter AN Arabic number B paragraph separator BN boundary neutral CS common separator EN European number ES European separator ET European terminator FSI first strong isolate L left-to-right LRE left-to-right embedding LRI left-to-right isolate LRO left-to-right override NSM non-spacing mark ON other neutral PDF pop directional format PDI pop directional isolate R right-to-left RLE right-to-left embedding RLI right-to-left isolate RLO right-to-left override S segment separator WS which space
CHARACTER CLASSES
[...] positive character class [^...] negative character class [x-y] range (can be used for hex characters) [[:xxx:]] positive POSIX named set [[:^xxx:]] negative POSIX named set
alnum alphanumeric alpha alphabetic ascii 0-127 blank space or tab cntrl control character digit decimal digit graph printing, excluding space lower lower case letter print printing, including space punct printing, excluding alphanumeric space white space upper upper case letter word same as \w xdigit hexadecimal digit
In PCRE2, POSIX character set names recognize only ASCII characters by default, but some of them use Unicode properties if PCRE2_UCP is set. You can use \Q...\E inside a character class.
QUANTIFIERS
? 0 or 1, greedy ?+ 0 or 1, possessive ?? 0 or 1, lazy * 0 or more, greedy *+ 0 or more, possessive *? 0 or more, lazy + 1 or more, greedy ++ 1 or more, possessive +? 1 or more, lazy {n} exactly n {n,m} at least n, no more than m, greedy {n,m}+ at least n, no more than m, possessive {n,m}? at least n, no more than m, lazy {n,} n or more, greedy {n,}+ n or more, possessive {n,}? n or more, lazy {,m} zero up to m, greedy {,m}+ zero up to m, possessive {,m}? zero up to m, lazy
ANCHORS AND SIMPLE ASSERTIONS
\b word boundary \B not a word boundary ^ start of subject also after an internal newline in multiline mode (after any newline if PCRE2_ALT_CIRCUMFLEX is set) \A start of subject $ end of subject also before newline at end of subject also before internal newline in multiline mode \Z end of subject also before newline at end of subject \z end of subject \G first matching position in subject
REPORTED MATCH POINT SETTING
\K set reported start of match
From release 10.38 \K is not permitted by default in lookaround asser- tions, for compatibility with Perl. However, if the PCRE2_EXTRA_AL- LOW_LOOKAROUND_BSK option is set, the previous behaviour is re-enabled. When this option is set, \K is honoured in positive assertions, but ig- nored in negative ones.
ALTERNATION
expr|expr|expr...
CAPTURING
(...) capture group (?<name>...) named capture group (Perl) (?'name'...) named capture group (Perl) (?P<name>...) named capture group (Python) (?:...) non-capture group (?|...) non-capture group; reset group numbers for capture groups in each alternative
In non-UTF modes, names may contain underscores and ASCII letters and digits; in UTF modes, any Unicode letters and Unicode decimal digits are permitted. In both cases, a name must not start with a digit.
ATOMIC GROUPS
(?>...) atomic non-capture group (*atomic:...) atomic non-capture group
COMMENT
(?#....) comment (not nestable)
OPTION SETTING Changes of these options within a group are automatically cancelled at the end of the group.
(?a) all ASCII options (?aD) restrict \d to ASCII in UCP mode (?aS) restrict \s to ASCII in UCP mode (?aW) restrict \w to ASCII in UCP mode (?aP) restrict all POSIX classes to ASCII in UCP mode (?aT) restrict POSIX digit classes to ASCII in UCP mode (?i) caseless (?J) allow duplicate named groups (?m) multiline (?n) no auto capture (?r) restrict caseless to either ASCII or non-ASCII (?s) single line (dotall) (?U) default ungreedy (lazy) (?x) ignore white space except in classes or \Q...\E (?xx) as (?x) but also ignore space and tab in classes (?-...) unset the given option(s) (?^) unset imnrsx options
(?aP) implies (?aT) as well, though this has no additional effect. How- ever, it means that (?-aP) is really (?-PT) which disables all ASCII restrictions for POSIX classes.
Unsetting x or xx unsets both. Several options may be set at once, and a mixture of setting and unsetting such as (?i-x) is allowed, but there may be only one hyphen. Setting (but no unsetting) is allowed after (?^ for example (?^in). An option setting may appear at the start of a non- capture group, for example (?i:...).
The following are recognized only at the very start of a pattern or af- ter one of the newline or \R options with similar syntax. More than one of them may appear. For the first three, d is a decimal number.
(*LIMIT_DEPTH=d) set the backtracking limit to d (*LIMIT_HEAP=d) set the heap size limit to d * 1024 bytes (*LIMIT_MATCH=d) set the match limit to d (*NOTEMPTY) set PCRE2_NOTEMPTY when matching (*NOTEMPTY_ATSTART) set PCRE2_NOTEMPTY_ATSTART when matching (*NO_AUTO_POSSESS) no auto-possessification (PCRE2_NO_AUTO_POSSESS) (*NO_DOTSTAR_ANCHOR) no .* anchoring (PCRE2_NO_DOTSTAR_ANCHOR) (*NO_JIT) disable JIT optimization (*NO_START_OPT) no start-match optimization (PCRE2_NO_START_OPTIMIZE) (*UTF) set appropriate UTF mode for the library in use (*UCP) set PCRE2_UCP (use Unicode properties for \d etc)
Note that LIMIT_DEPTH, LIMIT_HEAP, and LIMIT_MATCH can only reduce the value of the limits set by the caller of pcre2_match() or pcre2_dfa_match(), not increase them. LIMIT_RECURSION is an obsolete synonym for LIMIT_DEPTH. The application can lock out the use of (*UTF) and (*UCP) by setting the PCRE2_NEVER_UTF or PCRE2_NEVER_UCP options, respectively, at compile time.
NEWLINE CONVENTION
These are recognized only at the very start of the pattern or after op- tion settings with a similar syntax.
(*CR) carriage return only (*LF) linefeed only (*CRLF) carriage return followed by linefeed (*ANYCRLF) all three of the above (*ANY) any Unicode newline sequence (*NUL) the NUL character (binary zero)
WHAT \R MATCHES
These are recognized only at the very start of the pattern or after op- tion setting with a similar syntax.
(*BSR_ANYCRLF) CR, LF, or CRLF (*BSR_UNICODE) any Unicode newline sequence
LOOKAHEAD AND LOOKBEHIND ASSERTIONS
(?=...) ) (*pla:...) ) positive lookahead (*positive_lookahead:...) )
(?!...) ) (*nla:...) ) negative lookahead (*negative_lookahead:...) )
(?<=...) ) (*plb:...) ) positive lookbehind (*positive_lookbehind:...) )
(?<!...) ) (*nlb:...) ) negative lookbehind (*negative_lookbehind:...) )
Each top-level branch of a lookbehind must have a limit for the number of characters it matches. If any branch can match a variable number of characters, the maximum for each branch is limited to a value set by the caller of pcre2_compile() or defaulted. The default is set when PCRE2 is built (ultimate default 255). If every branch matches a fixed number of characters, the limit for each branch is 65535 characters.
NON-ATOMIC LOOKAROUND ASSERTIONS
These assertions are specific to PCRE2 and are not Perl-compatible.
(?*...) ) (*napla:...) ) synonyms (*non_atomic_positive_lookahead:...) )
(?<*...) ) (*naplb:...) ) synonyms (*non_atomic_positive_lookbehind:...) )
SCRIPT RUNS
(*script_run:...) ) script run, can be backtracked into (*sr:...) )
(*atomic_script_run:...) ) atomic script run (*asr:...) )
BACKREFERENCES
\n reference by number (can be ambiguous) \gn reference by number \g{n} reference by number \g+n relative reference by number (PCRE2 extension) \g-n relative reference by number \g{+n} relative reference by number (PCRE2 extension) \g{-n} relative reference by number \k<name> reference by name (Perl) \k'name' reference by name (Perl) \g{name} reference by name (Perl) \k{name} reference by name (.NET) (?P=name) reference by name (Python)
SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)
(?R) recurse whole pattern (?n) call subroutine by absolute number (?+n) call subroutine by relative number (?-n) call subroutine by relative number (?&name) call subroutine by name (Perl) (?P>name) call subroutine by name (Python) \g<name> call subroutine by name (Oniguruma) \g'name' call subroutine by name (Oniguruma) \g<n> call subroutine by absolute number (Oniguruma) \g'n' call subroutine by absolute number (Oniguruma) \g<+n> call subroutine by relative number (PCRE2 extension) \g'+n' call subroutine by relative number (PCRE2 extension) \g<-n> call subroutine by relative number (PCRE2 extension) \g'-n' call subroutine by relative number (PCRE2 extension)
CONDITIONAL PATTERNS
(?(condition)yes-pattern) (?(condition)yes-pattern|no-pattern)
(?(n) absolute reference condition (?(+n) relative reference condition (PCRE2 extension) (?(-n) relative reference condition (PCRE2 extension) (?(<name>) named reference condition (Perl) (?('name') named reference condition (Perl) (?(name) named reference condition (PCRE2, deprecated) (?(R) overall recursion condition (?(Rn) specific numbered group recursion condition (?(R&name) specific named group recursion condition (?(DEFINE) define groups for reference (?(VERSION[>]=n.m) test PCRE2 version (?(assert) assertion condition
Note the ambiguity of (?(R) and (?(Rn) which might be named reference conditions or recursion tests. Such a condition is interpreted as a reference condition if the relevant named group exists.
BACKTRACKING CONTROL
All backtracking control verbs may be in the form (*VERB:NAME). For (*MARK) the name is mandatory, for the others it is optional. (*SKIP) changes its behaviour if :NAME is present. The others just set a name for passing back to the caller, but this is not a name that (*SKIP) can see. The following act immediately they are reached:
(*ACCEPT) force successful match (*FAIL) force backtrack; synonym (*F) (*MARK:NAME) set name to be passed back; synonym (*:NAME)
The following act only when a subsequent match failure causes a back- track to reach them. They all force a match failure, but they differ in what happens afterwards. Those that advance the start-of-match point do so only if the pattern is not anchored.
(*COMMIT) overall failure, no advance of starting point (*PRUNE) advance to next starting character (*SKIP) advance to current matching position (*SKIP:NAME) advance to position corresponding to an earlier (*MARK:NAME); if not found, the (*SKIP) is ignored (*THEN) local failure, backtrack to next alternation
The effect of one of these verbs in a group called as a subroutine is confined to the subroutine call.
CALLOUTS
(?C) callout (assumed number 0) (?Cn) callout with numerical data n (?C"text") callout with string data
The allowed string delimiters are ` ' " ^ % # $ (which are the same for the start and the end), and the starting delimiter { matched with the ending delimiter }. To encode the ending delimiter within the string, double it.
SEE ALSO
pcre2pattern(3), pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2(3).
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
REVISION
Last updated: 12 October 2023 Copyright (c) 1997-2023 University of Cambridge.
PCRE2 10.43 12 October 2023 PCRE2SYNTAX(3)------------------------------------------------------------------------------
PCRE2UNICODE(3) Library Functions Manual PCRE2UNICODE(3)
NAME PCRE - Perl-compatible regular expressions (revised API)
UNICODE AND UTF SUPPORT
PCRE2 is normally built with Unicode support, though if you do not need it, you can build it without, in which case the library will be smaller. With Unicode support, PCRE2 has knowledge of Unicode character properties and can process strings of text in UTF-8, UTF-16, and UTF-32 format (depending on the code unit width), but this is not the default. Unless specifically requested, PCRE2 treats each code unit in a string as one character.
There are two ways of telling PCRE2 to switch to UTF mode, where char- acters may consist of more than one code unit and the range of values is constrained. The program can call pcre2_compile() with the PCRE2_UTF option, or the pattern may start with the sequence (*UTF). However, the latter facility can be locked out by the PCRE2_NEVER_UTF option. That is, the programmer can prevent the supplier of the pattern from switching to UTF mode.
Note that the PCRE2_MATCH_INVALID_UTF option (see below) forces PCRE2_UTF to be set.
In UTF mode, both the pattern and any subject strings that are matched against it are treated as UTF strings instead of strings of individual one-code-unit characters. There are also some other changes to the way characters are handled, as documented below.
UNICODE PROPERTY SUPPORT
When PCRE2 is built with Unicode support, the escape sequences \p{..}, \P{..}, and \X can be used. This is not dependent on the PCRE2_UTF set- ting. The Unicode properties that can be tested are a subset of those that Perl supports. Currently they are limited to the general category properties such as Lu for an upper case letter or Nd for a decimal num- ber, the derived properties Any and LC (synonym L&), the Unicode script names such as Arabic or Han, Bidi_Class, Bidi_Control, and a few binary properties.
The full lists are given in the pcre2pattern and pcre2syntax documenta- tion. In general, only the short names for properties are supported. For example, \p{L} matches a letter. Its longer synonym, \p{Letter}, is not supported. Furthermore, in Perl, many properties may optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE2 does not sup- port this.
WIDE CHARACTERS AND UTF MODES
Code points less than 256 can be specified in patterns by either braced or unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3). Larger values have to use braced sequences. Unbraced octal code points up to \777 are also recognized; larger ones can be coded using \o{...}.
The escape sequence \N{U+<hex digits>} is recognized as another way of specifying a Unicode character by code point in a UTF mode. It is not allowed in non-UTF mode.
In UTF mode, repeat quantifiers apply to complete UTF characters, not to individual code units.
In UTF mode, the dot metacharacter matches one UTF character instead of a single code unit.
In UTF mode, capture group names are not restricted to ASCII, and may contain any Unicode letters and decimal digits, as well as underscore.
The escape sequence \C can be used to match a single code unit in UTF mode, but its use can lead to some strange effects because it breaks up multi-unit characters (see the description of \C in the pcre2pattern documentation). For this reason, there is a build-time option that dis- ables support for \C completely. There is also a less draconian com- pile-time option for locking out the use of \C when a pattern is com- piled.
The use of \C is not supported by the alternative matching function pcre2_dfa_match() when in UTF-8 or UTF-16 mode, that is, when a charac- ter may consist of more than one code unit. The use of \C in these modes provokes a match-time error. Also, the JIT optimization does not support \C in these modes. If JIT optimization is requested for a UTF-8 or UTF-16 pattern that contains \C, it will not succeed, and so when pcre2_match() is called, the matching will be carried out by the inter- pretive function.
The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test characters of any code value, but, by default, the characters that PCRE2 recognizes as digits, spaces, or word characters remain the same set as in non-UTF mode, all with code points less than 256. This re- mains true even when PCRE2 is built to include Unicode support, because to do otherwise would slow down matching in many common cases. Note that this also applies to \b and \B, because they are defined in terms of \w and \W. If you want to test for a wider sense of, say, "digit", you can use explicit Unicode property tests such as \p{Nd}. Alterna- tively, if you set the PCRE2_UCP option, the way that the character es- capes work is changed so that Unicode properties are used to determine which characters match, though there are some options that suppress this for individual escapes. For details see the section on generic character types in the pcre2pattern documentation.
Like the escapes, characters that match the POSIX named character classes are all low-valued characters unless the PCRE2_UCP option is set, but there is an option to override this.
In contrast to the character escapes and character classes, the special horizontal and vertical white space escapes (\h, \H, \v, and \V) do match all the appropriate Unicode characters, whether or not PCRE2_UCP is set.
UNICODE CASE-EQUIVALENCE
If either PCRE2_UTF or PCRE2_UCP is set, upper/lower case processing makes use of Unicode properties except for characters whose code points are less than 128 and that have at most two case-equivalent values. For these, a direct table lookup is used for speed. A few Unicode charac- ters such as Greek sigma have more than two code points that are case- equivalent, and these are treated specially. Setting PCRE2_UCP without PCRE2_UTF allows Unicode-style case processing for non-UTF character encodings such as UCS-2.
There are two ASCII characters (S and K) that, in addition to their ASCII lower case equivalents, have a non-ASCII one as well (long S and Kelvin sign). Recognition of these non-ASCII characters as case-equiv- alent to their ASCII counterparts can be disabled by setting the PCRE2_EXTRA_CASELESS_RESTRICT option. When this is set, all characters in a case equivalence must either be ASCII or non-ASCII; there can be no mixing.
SCRIPT RUNS
The pattern constructs (*script_run:...) and (*atomic_script_run:...), with synonyms (*sr:...) and (*asr:...), verify that the string matched within the parentheses is a script run. In concept, a script run is a sequence of characters that are all from the same Unicode script. How- ever, because some scripts are commonly used together, and because some diacritical and other marks are used with multiple scripts, it is not that simple.
Every Unicode character has a Script property, mostly with a value cor- responding to the name of a script, such as Latin, Greek, or Cyrillic. There are also three special values:
"Unknown" is used for code points that have not been assigned, and also for the surrogate code points. In the PCRE2 32-bit library, characters whose code points are greater than the Unicode maximum (U+10FFFF), which are accessible only in non-UTF mode, are assigned the Unknown script.
"Common" is used for characters that are used with many scripts. These include punctuation, emoji, mathematical, musical, and currency sym- bols, and the ASCII digits 0 to 9.
"Inherited" is used for characters such as diacritical marks that mod- ify a previous character. These are considered to take on the script of the character that they modify.
Some Inherited characters are used with many scripts, but many of them are only normally used with a small number of scripts. For example, U+102E0 (Coptic Epact thousands mark) is used only with Arabic and Cop- tic. In order to make it possible to check this, a Unicode property called Script Extension exists. Its value is a list of scripts that ap- ply to the character. For the majority of characters, the list contains just one script, the same one as the Script property. However, for characters such as U+102E0 more than one Script is listed. There are also some Common characters that have a single, non-Common script in their Script Extension list.
The next section describes the basic rules for deciding whether a given string of characters is a script run. Note, however, that there are some special cases involving the Chinese Han script, and an additional constraint for decimal digits. These are covered in subsequent sec- tions.
Basic script run rules
A string that is less than two characters long is a script run. This is the only case in which an Unknown character can be part of a script run. Longer strings are checked using only the Script Extensions prop- erty, not the basic Script property.
If a character's Script Extension property is the single value "Inher- ited", it is always accepted as part of a script run. This is also true for the property "Common", subject to the checking of decimal digits described below. All the remaining characters in a script run must have at least one script in common in their Script Extension lists. In set- theoretic terminology, the intersection of all the sets of scripts must not be empty.
A simple example is an Internet name such as "google.com". The letters are all in the Latin script, and the dot is Common, so this string is a script run. However, the Cyrillic letter "o" looks exactly the same as the Latin "o"; a string that looks the same, but with Cyrillic "o"s is not a script run.
More interesting examples involve characters with more than one script in their Script Extension. Consider the following characters:
U+060C Arabic comma U+06D4 Arabic full stop
The first has the Script Extension list Arabic, Hanifi Rohingya, Syr- iac, and Thaana; the second has just Arabic and Hanifi Rohingya. Both of them could appear in script runs of either Arabic or Hanifi Ro- hingya. The first could also appear in Syriac or Thaana script runs, but the second could not.
The Chinese Han script
The Chinese Han script is commonly used in conjunction with other scripts for writing certain languages. Japanese uses the Hiragana and Katakana scripts together with Han; Korean uses Hangul and Han; Tai- wanese Mandarin uses Bopomofo and Han. These three combinations are treated as special cases when checking script runs and are, in effect, "virtual scripts". Thus, a script run may contain a mixture of Hira- gana, Katakana, and Han, or a mixture of Hangul and Han, or a mixture of Bopomofo and Han, but not, for example, a mixture of Hangul and Bopomofo and Han. PCRE2 (like Perl) follows Unicode's Technical Stan- dard 39 ("Unicode Security Mechanisms", http://unicode.org/re- ports/tr39/) in allowing such mixtures.
Decimal digits
Unicode contains many sets of 10 decimal digits in different scripts, and some scripts (including the Common script) contain more than one set. Some of these decimal digits them are visually indistinguishable from the common ASCII digits. In addition to the script checking de- scribed above, if a script run contains any decimal digits, they must all come from the same set of 10 adjacent characters.
VALIDITY OF UTF STRINGS
When the PCRE2_UTF option is set, the strings passed as patterns and subjects are (by default) checked for validity on entry to the relevant functions. If an invalid UTF string is passed, a negative error code is returned. The code unit offset to the offending character can be ex- tracted from the match data block by calling pcre2_get_startchar(), which is used for this purpose after a UTF error.
In some situations, you may already know that your strings are valid, and therefore want to skip these checks in order to improve perfor- mance, for example in the case of a long subject string that is being scanned repeatedly. If you set the PCRE2_NO_UTF_CHECK option at com- pile time or at match time, PCRE2 assumes that the pattern or subject it is given (respectively) contains only valid UTF code unit sequences.
If you pass an invalid UTF string when PCRE2_NO_UTF_CHECK is set, the result is undefined and your program may crash or loop indefinitely or give incorrect results. There is, however, one mode of matching that can handle invalid UTF subject strings. This is enabled by passing PCRE2_MATCH_INVALID_UTF to pcre2_compile() and is discussed below in the next section. The rest of this section covers the case when PCRE2_MATCH_INVALID_UTF is not set.
Passing PCRE2_NO_UTF_CHECK to pcre2_compile() just disables the UTF check for the pattern; it does not also apply to subject strings. If you want to disable the check for a subject string you must pass this same option to pcre2_match() or pcre2_dfa_match().
UTF-16 and UTF-32 strings can indicate their endianness by special code knows as a byte-order mark (BOM). The PCRE2 functions do not handle this, expecting strings to be in host byte order.
Unless PCRE2_NO_UTF_CHECK is set, a UTF string is checked before any other processing takes place. In the case of pcre2_match() and pcre2_dfa_match() calls with a non-zero starting offset, the check is applied only to that part of the subject that could be inspected during matching, and there is a check that the starting offset points to the first code unit of a character or to the end of the subject. If there are no lookbehind assertions in the pattern, the check starts at the starting offset. Otherwise, it starts at the length of the longest lookbehind before the starting offset, or at the start of the subject if there are not that many characters before the starting offset. Note that the sequences \b and \B are one-character lookbehinds.
In addition to checking the format of the string, there is a check to ensure that all code points lie in the range U+0 to U+10FFFF, excluding the surrogate area. The so-called "non-character" code points are not excluded because Unicode corrigendum #9 makes it clear that they should not be.
Characters in the "Surrogate Area" of Unicode are reserved for use by UTF-16, where they are used in pairs to encode code points with values greater than 0xFFFF. The code points that are encoded by UTF-16 pairs are available independently in the UTF-8 and UTF-32 encodings. (In other words, the whole surrogate thing is a fudge for UTF-16 which un- fortunately messes up UTF-8 and UTF-32.)
Setting PCRE2_NO_UTF_CHECK at compile time does not disable the error that is given if an escape sequence for an invalid Unicode code point is encountered in the pattern. If you want to allow escape sequences such as \x{d800} (a surrogate code point) you can set the PCRE2_EX- TRA_ALLOW_SURROGATE_ESCAPES extra option. However, this is possible only in UTF-8 and UTF-32 modes, because these values are not repre- sentable in UTF-16.
Errors in UTF-8 strings
The following negative error codes are given for invalid UTF-8 strings:
PCRE2_ERROR_UTF8_ERR1 PCRE2_ERROR_UTF8_ERR2 PCRE2_ERROR_UTF8_ERR3 PCRE2_ERROR_UTF8_ERR4 PCRE2_ERROR_UTF8_ERR5
The string ends with a truncated UTF-8 character; the code specifies how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8 characters to be no longer than 4 bytes, the encoding scheme (origi- nally defined by RFC 2279) allows for up to 6 bytes, and this is checked first; hence the possibility of 4 or 5 missing bytes.
PCRE2_ERROR_UTF8_ERR6 PCRE2_ERROR_UTF8_ERR7 PCRE2_ERROR_UTF8_ERR8 PCRE2_ERROR_UTF8_ERR9 PCRE2_ERROR_UTF8_ERR10
The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of the character do not have the binary value 0b10 (that is, either the most significant bit is 0, or the next bit is 1).
PCRE2_ERROR_UTF8_ERR11 PCRE2_ERROR_UTF8_ERR12
A character that is valid by the RFC 2279 rules is either 5 or 6 bytes long; these code points are excluded by RFC 3629.
PCRE2_ERROR_UTF8_ERR13
A 4-byte character has a value greater than 0x10ffff; these code points are excluded by RFC 3629.
PCRE2_ERROR_UTF8_ERR14
A 3-byte character has a value in the range 0xd800 to 0xdfff; this range of code points are reserved by RFC 3629 for use with UTF-16, and so are excluded from UTF-8.
PCRE2_ERROR_UTF8_ERR15 PCRE2_ERROR_UTF8_ERR16 PCRE2_ERROR_UTF8_ERR17 PCRE2_ERROR_UTF8_ERR18 PCRE2_ERROR_UTF8_ERR19
A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes for a value that can be represented by fewer bytes, which is invalid. For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor- rect coding uses just one byte.
PCRE2_ERROR_UTF8_ERR20
The two most significant bits of the first byte of a character have the binary value 0b10 (that is, the most significant bit is 1 and the sec- ond is 0). Such a byte can only validly occur as the second or subse- quent byte of a multi-byte character.
PCRE2_ERROR_UTF8_ERR21
The first byte of a character has the value 0xfe or 0xff. These values can never occur in a valid UTF-8 string.
Errors in UTF-16 strings
The following negative error codes are given for invalid UTF-16 strings:
PCRE2_ERROR_UTF16_ERR1 Missing low surrogate at end of string PCRE2_ERROR_UTF16_ERR2 Invalid low surrogate follows high surrogate PCRE2_ERROR_UTF16_ERR3 Isolated low surrogate
Errors in UTF-32 strings
The following negative error codes are given for invalid UTF-32 strings:
PCRE2_ERROR_UTF32_ERR1 Surrogate character (0xd800 to 0xdfff) PCRE2_ERROR_UTF32_ERR2 Code point is greater than 0x10ffff
MATCHING IN INVALID UTF STRINGS
You can run pattern matches on subject strings that may contain invalid UTF sequences if you call pcre2_compile() with the PCRE2_MATCH_IN- VALID_UTF option. This is supported by pcre2_match(), including JIT matching, but not by pcre2_dfa_match(). When PCRE2_MATCH_INVALID_UTF is set, it forces PCRE2_UTF to be set as well. Note, however, that the pattern itself must be a valid UTF string.
If you do not set PCRE2_MATCH_INVALID_UTF when calling pcre2_compile, and you are not certain that your subject strings are valid UTF se- quences, you should not make use of the JIT "fast path" function pcre2_jit_match() because it bypasses sanity checks, including the one for UTF validity. An invalid string may cause undefined behaviour, in- cluding looping, crashing, or giving the wrong answer.
Setting PCRE2_MATCH_INVALID_UTF does not affect what pcre2_compile() generates, but if pcre2_jit_compile() is subsequently called, it does generate different code. If JIT is not used, the option affects the be- haviour of the interpretive code in pcre2_match(). When PCRE2_MATCH_IN- VALID_UTF is set at compile time, PCRE2_NO_UTF_CHECK is ignored at match time.
In this mode, an invalid code unit sequence in the subject never matches any pattern item. It does not match dot, it does not match \p{Any}, it does not even match negative items such as [^X]. A lookbe- hind assertion fails if it encounters an invalid sequence while moving the current point backwards. In other words, an invalid UTF code unit sequence acts as a barrier which no match can cross.
You can also think of this as the subject being split up into fragments of valid UTF, delimited internally by invalid code unit sequences. The pattern is matched fragment by fragment. The result of a successful match, however, is given as code unit offsets in the entire subject string in the usual way. There are a few points to consider:
The internal boundaries are not interpreted as the beginnings or ends of lines and so do not match circumflex or dollar characters in the pattern.
If pcre2_match() is called with an offset that points to an invalid UTF-sequence, that sequence is skipped, and the match starts at the next valid UTF character, or the end of the subject.
At internal fragment boundaries, \b and \B behave in the same way as at the beginning and end of the subject. For example, a sequence such as \bWORD\b would match an instance of WORD that is surrounded by invalid UTF code units.
Using PCRE2_MATCH_INVALID_UTF, an application can run matches on arbi- trary data, knowing that any matched strings that are returned are valid UTF. This can be useful when searching for UTF text in executable or other binary files.
Note, however, that the 16-bit and 32-bit PCRE2 libraries process strings as sequences of uint16_t or uint32_t code points. They cannot find valid UTF sequences within an arbitrary string of bytes unless such sequences are suitably aligned.
AUTHOR
Philip Hazel Retired from University Computing Service Cambridge, England.
REVISION
Last updated: 12 October 2023 Copyright (c) 1997-2023 University of Cambridge.
PCRE2 10.43 04 February 2023 PCRE2UNICODE(3)------------------------------------------------------------------------------
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