github/doc/pcre2pattern.3


								.TH PCRE2PATTERN 3 "04 June 2024" "PCRE2 10.44"

								.SH NAME

								PCRE2 - Perl-compatible regular expressions (revised API)

								.SH "PCRE2 REGULAR EXPRESSION DETAILS"

								.rs

								.sp

								The syntax and semantics of the regular expressions that are supported by PCRE2

								are described in detail below. There is a quick-reference syntax summary in the

								.\" HREF

								\fBpcre2syntax\fP

								.\"

								page. PCRE2 tries to match Perl syntax and semantics as closely as it can.

								PCRE2 also supports some alternative 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.

								.P

								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 Expressions", published

								by O'Reilly, covers regular expressions in great detail. This description of

								PCRE2's regular expressions is intended as reference material.

								.P

								This document discusses the regular expression patterns that are supported by

								PCRE2 when its main matching function, \fBpcre2_match()\fP, is used. PCRE2 also

								has an alternative matching function, \fBpcre2_dfa_match()\fP, 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

								function, are discussed in the

								.\" HREF

								\fBpcre2matching\fP

								.\"

								page.

								.

								.

								.SH "SPECIAL START-OF-PATTERN ITEMS"

								.rs

								.sp

								A number of options that can be passed to \fBpcre2_compile()\fP can also be set

								by special items at the start of a pattern. These are not Perl-compatible, but

								are provided to make these options accessible to pattern writers who are not

								able to change the program that processes the pattern. Any number of these

								items may appear, but they must all be together right at the start of the

								pattern string, and the letters must be in upper case.

								.

								.

								.SS "UTF support"

								.rs

								.sp

								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

								.\" HREF

								\fBpcre2unicode\fP

								.\"

								page.

								.P

								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 \fBpcre2_compile()\fP, (*UTF) is not allowed, and its

								appearance in a pattern causes an error.

								.

								.

								.SS "Unicode property support"

								.rs

								.sp

								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 \ed and \ew to use Unicode properties to determine character types,

								instead of recognizing only characters with codes less than 256 via a lookup

								table. If also causes upper/lower casing operations 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

								.\" HTML <a href="#internaloptions">

								.\" </a>

								"Internal Option Setting"

								.\"

								below.

								.P

								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

								\fBpcre2_compile()\fP, (*UCP) is not allowed, and its appearance in a pattern

								causes an error.

								.

								.

								.SS "Locking out empty string matching"

								.rs

								.sp

								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 pattern. These options

								lock out the matching of empty strings, either entirely, or only at the start

								of the subject.

								.

								.

								.SS "Disabling auto-possessification"

								.rs

								.sp

								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

								.\" HREF

								\fBpcre2api\fP

								.\"

								documentation.

								.

								.

								.SS "Disabling start-up optimizations"

								.rs

								.sp

								If a pattern starts with (*NO_START_OPT), it has the same effect as setting the

								PCRE2_NO_START_OPTIMIZE option. This disables several optimizations for quickly

								reaching "no match" results. For more details, see the

								.\" HREF

								\fBpcre2api\fP

								.\"

								documentation.

								.

								.

								.SS "Disabling automatic anchoring"

								.rs

								.sp

								If a pattern starts with (*NO_DOTSTAR_ANCHOR), it has the same effect as

								setting the PCRE2_NO_DOTSTAR_ANCHOR option. This disables optimizations that

								apply to patterns whose top-level branches all start with .* (match any number

								of arbitrary characters). For more details, see the

								.\" HREF

								\fBpcre2api\fP

								.\"

								documentation.

								.

								.

								.SS "Disabling JIT compilation"

								.rs

								.sp

								If a pattern that starts with (*NO_JIT) is successfully compiled, an attempt by

								the application to apply the JIT optimization by calling

								\fBpcre2_jit_compile()\fP is ignored.

								.

								.

								.SS "Setting match resource limits"

								.rs

								.sp

								The \fBpcre2_match()\fP function contains a counter that is incremented every

								time it goes round its main loop. The caller of \fBpcre2_match()\fP 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.

								.P

								These facilities are provided to catch runaway matches that are provoked by

								patterns with huge matching trees. A common example is a pattern with nested

								unlimited repeats applied to a long string that does not match. When one of

								these limits is reached, \fBpcre2_match()\fP gives an error return. The limits

								can also be set by items at the start of the pattern of the form

								.sp

								  (*LIMIT_HEAP=d)

								  (*LIMIT_MATCH=d)

								  (*LIMIT_DEPTH=d)

								.sp

								where d is any number of decimal digits. However, the value of the setting must

								be less than the value set (or defaulted) by the caller of \fBpcre2_match()\fP

								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).

								.P

								Prior to release 10.30, LIMIT_DEPTH was called LIMIT_RECURSION. This name is

								still recognized for backwards compatibility.

								.P

								The heap limit applies only when the \fBpcre2_match()\fP or

								\fBpcre2_dfa_match()\fP 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 \fBpcre2_dfa_match()\fP 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.

								.

								.

								.\" HTML <a name="newlines"></a>

								.SS "Newline conventions"

								.rs

								.sp

								PCRE2 supports six different conventions for indicating line breaks in

								strings: a single CR (carriage return) character, a single LF (linefeed)

								character, the two-character sequence CRLF, any of the three preceding, any

								Unicode newline sequence, or the NUL character (binary zero). The

								.\" HREF

								\fBpcre2api\fP

								.\"

								page has

								.\" HTML <a href="pcre2api.html#newlines">

								.\" </a>

								further discussion

								.\"

								about newlines, and shows how to set the newline convention when calling

								\fBpcre2_compile()\fP.

								.P

								It is also possible to specify a newline convention by starting a pattern

								string with one of the following sequences:

								.sp

								  (*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)

								.sp

								These override the default and the options given to the compiling function. For

								example, on a Unix system where LF is the default newline sequence, the pattern

								.sp

								  (*CR)a.b

								.sp

								changes the convention to CR. That pattern matches "a\enb" because LF is no

								longer a newline. If more than one of these settings is present, the last one

								is used.

								.P

								The newline convention affects where the circumflex and dollar assertions are

								true. It also affects the interpretation of the dot metacharacter when

								PCRE2_DOTALL is not set, and the behaviour of \eN when not followed by an

								opening brace. However, it does not affect what the \eR escape sequence

								matches. By default, this is any Unicode newline sequence, for Perl

								compatibility. However, this can be changed; see the next section and the

								description of \eR in the section entitled

								.\" HTML <a href="#newlineseq">

								.\" </a>

								"Newline sequences"

								.\"

								below. A change of \eR setting can be combined with a change of newline

								convention.

								.

								.

								.SS "Specifying what \eR matches"

								.rs

								.sp

								It is possible to restrict \eR 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_UNICODE) is also recognized,

								corresponding to PCRE2_BSR_UNICODE.

								.

								.

								.SH "EBCDIC CHARACTER CODES"

								.rs

								.sp

								PCRE2 can be compiled to run in an environment that uses EBCDIC as its

								character code instead of ASCII or Unicode (typically a mainframe system). In

								the sections below, character code values are ASCII or Unicode; in an EBCDIC

								environment these characters may have different code values, and there are no

								code points greater than 255.

								.

								.

								.SH "CHARACTERS AND METACHARACTERS"

								.rs

								.sp

								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

								.sp

								  The quick brown fox

								.sp

								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

								\fBpcre2_compile()\fP or set by (?r) within the pattern).

								.P

								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 \fImetacharacters\fP, which do not stand

								for themselves but instead are interpreted in some special way.

								.P

								There are two different sets of metacharacters: those that are recognized

								anywhere in the pattern except within square brackets, and those that are

								recognized within square brackets. Outside square brackets, the metacharacters

								are as follows:

								.sp

								  \e      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

								.sp

								Brace characters { and } are also used to enclose data for constructions such

								as \eg{2} or \ek{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 before or after the comma. The

								exception to this is \eu{...} 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.

								.P

								Part of a pattern that is in square brackets is called a "character class". In

								a character class the only metacharacters are:

								.sp

								  \e      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

								.sp

								If a pattern is compiled with the PCRE2_EXTENDED option, most white space in

								the pattern, other than in a character class, within a \eQ...\eE sequence, or

								between a # outside a character class and the next newline, inclusive, are

								ignored. An escaping backslash can be used to include 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

								.\" HTML <a href="#internaloptions">

								.\" </a>

								"Internal Option Setting"

								.\"

								below.

								.P

								The following sections describe the use of each of the metacharacters.

								.

								.

								.SH BACKSLASH

								.rs

								.sp

								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.

								.P

								For example, if you want to match a * character, you must write \e* 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 backslash, you write \e\e.

								.P

								Only ASCII digits and letters have any special meaning after a backslash. All

								other characters (in particular, those whose code points are greater than 127)

								are treated as literals.

								.P

								If you want to treat all characters in a sequence as literals, you can do so by

								putting them between \eQ and \eE. 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

								\eQ...\eE sequences in PCRE2, whereas in Perl, $ and @ cause variable

								interpolation. Also, Perl does "double-quotish backslash interpolation" on any

								backslashes between \eQ and \eE which, its documentation says, "may lead to

								confusing results". PCRE2 treats a backslash between \eQ and \eE just like any

								other character. Note the following examples:

								.sp

								  Pattern            PCRE2 matches   Perl matches

								.sp

								.\" JOIN

								  \eQabc$xyz\eE        abc$xyz        abc followed by the

								                                      contents of $xyz

								  \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz

								  \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz

								  \eQA\eB\eE            A\eB            A\eB

								  \eQ\e\eE              \e              \e\eE

								.sp

								The \eQ...\eE sequence is recognized both inside and outside character classes.

								An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed

								by \eE later in the pattern, the literal interpretation continues to the end of

								the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside

								a character class, this causes an error, because the character class is then

								not terminated by a closing square bracket.

								.

								.

								.\" HTML <a name="digitsafterbackslash"></a>

								.SS "Non-printing characters"

								.rs

								.sp

								A second use of backslash provides a way of encoding non-printing characters

								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 represents. In an ASCII or Unicode

								environment, these escapes are as follows:

								.sp

								  \ea          alarm, that is, the BEL character (hex 07)

								  \ecx         "control-x", where x is a non-control ASCII character

								  \ee          escape (hex 1B)

								  \ef          form feed (hex 0C)

								  \en          linefeed (hex 0A)

								  \er          carriage return (hex 0D) (but see below)

								  \et          tab (hex 09)

								  \e0dd        character with octal code 0dd

								  \eddd        character with octal code ddd, or backreference

								  \eo{ddd..}   character with octal code ddd..

								  \exhh        character with hex code hh

								  \ex{hhh..}   character with hex code hhh..

								  \eN{U+hhh..} character with Unicode hex code point hhh..

								.sp

								By default, after \ex that is not followed by {, from zero to two hexadecimal

								digits are read (letters can be in upper or lower case). Any number of

								hexadecimal digits may appear between \ex{ and }. If a character other than a

								hexadecimal digit appears between \ex{ and }, or if there is no terminating },

								an error occurs.

								.P

								Characters whose code points are less than 256 can be defined by either of the

								two syntaxes for \ex or by an octal sequence. There is no difference in the way

								they are handled. For example, \exdc is exactly the same as \ex{dc} or \e334.

								However, using the braced versions does make such sequences easier to read.

								.P

								Support is available for some ECMAScript (aka JavaScript) escape sequences via

								two compile-time options. If PCRE2_ALT_BSUX is set, the sequence \ex followed

								by { is not recognized. Only if \ex is followed by two hexadecimal digits is it

								recognized as a character escape. Otherwise it is interpreted as a literal "x"

								character. In this mode, support for code points greater than 256 is provided

								by \eu, which must be followed by four hexadecimal digits; otherwise it is

								interpreted as a literal "u" character.

								.P

								PCRE2_EXTRA_ALT_BSUX has the same effect as PCRE2_ALT_BSUX and, in addition,

								\eu{hhh..} is recognized as the character specified by hexadecimal code point.

								There may be any number of hexadecimal digits, but unlike 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.

								.P

								The \eN{U+hhh..} escape sequence is recognized only when PCRE2 is operating in

								UTF mode. Perl also uses \eN{name} to specify characters by Unicode name; PCRE2

								does not support this. Note that when \eN is not followed by an opening brace

								(curly bracket) it has an entirely different meaning, matching any character

								that is not a newline.

								.P

								There are some legacy applications where the escape sequence \er is expected to

								match a newline. If the PCRE2_EXTRA_ESCAPED_CR_IS_LF option is set, \er in a

								pattern is converted to \en so that it matches a LF (linefeed) instead of a CR

								(carriage return) character.

								.P

								An error occurs if \ec is not followed by a character whose ASCII code point

								is in the range 32 to 126. The precise effect of \ecx is as follows: if x is a

								lower case letter, it is converted to upper case. Then bit 6 of the character

								(hex 40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is

								5A), but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If

								the code unit following \ec has a code point less than 32 or greater than 126,

								a compile-time error occurs.

								.P

								When PCRE2 is compiled in EBCDIC mode, \eN{U+hhh..} is not supported. \ea, \ee,

								\ef, \en, \er, and \et generate the appropriate EBCDIC code values. The \ec

								escape is processed as specified for Perl in the \fBperlebcdic\fP document. The

								only characters that are allowed after \ec are A-Z, a-z, or one of @, [, \e, ],

								^, _, or ?. Any other character provokes a compile-time error. The sequence

								\ec@ encodes character code 0; after \ec the letters (in either case) encode

								characters 1-26 (hex 01 to hex 1A); [, \e, ], ^, and _ encode characters 27-31

								(hex 1B to hex 1F), and \ec? becomes either 255 (hex FF) or 95 (hex 5F).

								.P

								Thus, apart from \ec?, 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, \ecG always generates code value 7, which is BEL in ASCII

								but DEL in EBCDIC.

								.P

								The sequence \ec? 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 \ec? generate 95; otherwise it generates 255.

								.P

								After \e0 up to two further octal digits are read. If there are fewer than two

								digits, just those that are present are used. Thus the sequence \e0\ex\e015

								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.

								.P

								The escape \eo 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.

								.P

								For greater clarity and unambiguity, it is best to avoid following \e by a

								digit greater than zero. Instead, use \eo{...} or \ex{...} to specify numerical

								character code points, and \eg{...} to specify backreferences. The following

								paragraphs describe the old, ambiguous syntax.

								.P

								The handling of a backslash followed by a digit other than 0 is complicated,

								and Perl has changed over time, causing PCRE2 also to change.

								.P

								Outside a character class, PCRE2 reads the digit and any following digits 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 \fIbackreference\fP. A description of how this

								works is given

								.\" HTML <a href="#backreferences">

								.\" </a>

								later,

								.\"

								following the discussion of

								.\" HTML <a href="#group">

								.\" </a>

								parenthesized groups.

								.\"

								Otherwise, up to three octal digits are read to form a character code.

								.P

								Inside a character class, PCRE2 handles \e8 and \e9 as the literal characters

								"8" and "9", and otherwise reads up to three octal digits following the

								backslash, using them to generate a data character. Any subsequent digits stand

								for themselves. For example, outside a character class:

								.sp

								  \e040   is another way of writing an ASCII space

								.\" JOIN

								  \e40    is the same, provided there are fewer than 40

								            previous capture groups

								  \e7     is always a backreference

								.\" JOIN

								  \e11    might be a backreference, or another way of

								            writing a tab

								  \e011   is always a tab

								  \e0113  is a tab followed by the character "3"

								.\" JOIN

								  \e113   might be a backreference, otherwise the

								            character with octal code 113

								.\" JOIN

								  \e377   might be a backreference, otherwise

								            the value 255 (decimal)

								  \e81    is always a backreference

								.sp

								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.

								.

								.

								.SS "Constraints on character values"

								.rs

								.sp

								Characters that are specified using octal or hexadecimal numbers are

								limited to certain values, as follows:

								.sp

								  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

								.sp

								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 \fBpcre2_compile()\fP 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.

								.

								.

								.SS "Escape sequences in character classes"

								.rs

								.sp

								All the sequences that define a single character value can be used both inside

								and outside character classes. In addition, inside a character class, \eb is

								interpreted as the backspace character (hex 08).

								.P

								When not followed by an opening brace, \eN is not allowed in a character class.

								\eB, \eR, and \eX 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.

								.

								.

								.SS "Unsupported escape sequences"

								.rs

								.sp

								In Perl, the sequences \eF, \el, \eL, \eu, and \eU 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 options is set, \eU matches a "U"

								character, and \eu can be used to define a character by code point, as

								described above.

								.

								.

								.SS "Absolute and relative backreferences"

								.rs

								.sp

								The sequence \eg followed by a signed or unsigned number, optionally enclosed

								in braces, is an absolute or relative backreference. A named backreference

								can be coded as \eg{name}. Backreferences are discussed

								.\" HTML <a href="#backreferences">

								.\" </a>

								later,

								.\"

								following the discussion of

								.\" HTML <a href="#group">

								.\" </a>

								parenthesized groups.

								.\"

								.

								.

								.SS "Absolute and relative subroutine calls"

								.rs

								.sp

								For compatibility with Oniguruma, the non-Perl syntax \eg 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

								.\" HTML <a href="#onigurumasubroutines">

								.\" </a>

								later.

								.\"

								Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP

								synonymous. The former is a backreference; the latter is a

								.\" HTML <a href="#groupsassubroutines">

								.\" </a>

								subroutine

								.\"

								call.

								.

								.

								.\" HTML <a name="genericchartypes"></a>

								.SS "Generic character types"

								.rs

								.sp

								Another use of backslash is for specifying generic character types:

								.sp

								  \ed     any decimal digit

								  \eD     any character that is not a decimal digit

								  \eh     any horizontal white space character

								  \eH     any character that is not a horizontal white space character

								  \eN     any character that is not a newline

								  \es     any white space character

								  \eS     any character that is not a white space character

								  \ev     any vertical white space character

								  \eV     any character that is not a vertical white space character

								  \ew     any "word" character

								  \eW     any "non-word" character

								.sp

								The \eN escape sequence has the same meaning as

								.\" HTML <a href="#fullstopdot">

								.\" </a>

								the "." metacharacter

								.\"

								when PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not change the

								meaning of \eN. Note that when \eN is followed by an opening brace it has a

								different meaning. See the section entitled

								.\" HTML <a href="#digitsafterbackslash">

								.\" </a>

								"Non-printing characters"

								.\"

								above for details. Perl also uses \eN{name} to specify characters by Unicode

								name; PCRE2 does not support this.

								.P

								Each pair of lower and upper case escape sequences partitions the complete 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.

								.P

								The default \es characters are HT (9), LF (10), VT (11), FF (12), CR (13), and

								space (32), which are defined as white space in the "C" locale. This list may

								vary if locale-specific matching is taking place. For example, in some locales

								the "non-breaking space" character (\exA0) is recognized as white space, and in

								others the VT character is not.

								.P

								A "word" character is an underscore or any character that is a letter or digit.

								By default, the definition of letters and digits is controlled by PCRE2's

								low-valued character tables, and may vary if locale-specific matching is taking

								place (see

								.\" HTML <a href="pcre2api.html#localesupport">

								.\" </a>

								"Locale support"

								.\"

								in the

								.\" HREF

								\fBpcre2api\fP

								.\"

								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 \ew. The use of locales with

								Unicode is discouraged.

								.P

								By default, characters whose code points are greater than 127 never match \ed,

								\es, or \ew, and always match \eD, \eS, and \eW, 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 efficiency reasons. If the PCRE2_UCP option

								is set, the behaviour is changed so that Unicode properties are used to

								determine character types, as follows:

								.sp

								  \ed  any character that matches \ep{Nd} (decimal digit)

								  \es  any character that matches \ep{Z} or \eh or \ev

								  \ew  any character that matches \ep{L}, \ep{N}, \ep{Mn}, or \ep{Pc}

								.sp

								The addition of \ep{Mn} (non-spacing mark) and the replacement of an explicit

								test for underscore with a test for \ep{Pc} (connector punctuation) happened in

								PCRE2 release 10.43. This brings PCRE2 into line with Perl.

								.P

								The upper case escapes match the inverse sets of characters. Note that \ed

								matches only decimal digits, whereas \ew matches any Unicode digit, as well as

								other character categories. Note also that PCRE2_UCP affects \eb, and

								\eB because they are defined in terms of \ew and \eW. Matching these sequences

								is noticeably slower when PCRE2_UCP is set.

								.P

								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

								.\" HTML <a href="#internaloptions">

								.\" </a>

								(see below).

								.\"

								.P

								The sequences \eh, \eH, \ev, and \eV, 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:

								.sp

								  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

								.sp

								The vertical space characters are:

								.sp

								  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

								.sp

								In 8-bit, non-UTF-8 mode, only the characters with code points less than 256

								are relevant.

								.

								.

								.\" HTML <a name="newlineseq"></a>

								.SS "Newline sequences"

								.rs

								.sp

								Outside a character class, by default, the escape sequence \eR matches any

								Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the

								following:

								.sp

								  (?>\er\en|\en|\ex0b|\ef|\er|\ex85)

								.sp

								This is an example of an "atomic group", details of which are given

								.\" HTML <a href="#atomicgroup">

								.\" </a>

								below.

								.\"

								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 (carriage 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.

								.P

								In other modes, two additional characters whose code points are greater than 255

								are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).

								Unicode support is not needed for these characters to be recognized.

								.P

								It is possible to restrict \eR 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 "backslash 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_UNICODE option. It is also possible to specify

								these settings by starting a pattern string with one of the following

								sequences:

								.sp

								  (*BSR_ANYCRLF)   CR, LF, or CRLF only

								  (*BSR_UNICODE)   any Unicode newline sequence

								.sp

								These override the default and the options given to the compiling function.

								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 example, a pattern can start with:

								.sp

								  (*ANY)(*BSR_ANYCRLF)

								.sp

								They can also be combined with the (*UTF) or (*UCP) special sequences. Inside a

								character class, \eR is treated as an unrecognized escape sequence, and causes

								an error.

								.

								.

								.\" HTML <a name="uniextseq"></a>

								.SS Unicode character properties

								.rs

								.sp

								When PCRE2 is built with Unicode support (the default), three additional 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 characters 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 Unknown script and with an unassigned type.

								.P

								Matching characters by Unicode property is not fast, because PCRE2 has to do a

								multistage table lookup in order to find a character's property. That is why

								the traditional escape sequences such as \ed and \ew 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).

								.P

								The extra escape sequences that provide property support are:

								.sp

								  \ep{\fIxx\fP}   a character with the \fIxx\fP property

								  \eP{\fIxx\fP}   a character without the \fIxx\fP property

								  \eX       a Unicode extended grapheme cluster

								.sp

								The property names represented by \fIxx\fP 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) properties, and some special

								PCRE2 properties (described

								.\" HTML <a href="#extraprops">

								.\" </a>

								below).

								.\"

								Certain other Perl properties such as "InMusicalSymbols" are not supported by

								PCRE2. Note that \eP{Any} does not match any characters, so always causes a

								match failure.

								.

								.

								.

								.SS "Script properties for \ep and \eP"

								.rs

								.sp

								There are three different syntax forms for matching a script. Each Unicode

								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, \ep{sc:Adlam} matches characters whose basic script is Adlam, whereas

								\ep{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,

								\ep{Adlam}, it is treated as \ep{scx:Adlam}. Perl changed to this

								interpretation at release 5.26 and PCRE2 changed at release 10.40.

								.P

								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 "Common". The current list

								of recognized script names and their 4-character abbreviations can be obtained

								by running this command:

								.sp

								  pcre2test -LS

								.sp

								.

								.

								.

								.SS "The general category property for \ep and \eP"

								.rs

								.sp

								Each character has exactly one Unicode general category property, specified by

								a two-letter abbreviation. For compatibility with Perl, negation can be

								specified by including a circumflex between the opening brace and the property

								name. For example, \ep{^Lu} is the same as \eP{Lu}.

								.P

								If only one letter is specified with \ep or \eP, it includes all the general

								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:

								.sp

								  \ep{L}

								  \epL

								.sp

								The following general category property codes are supported:

								.sp

								  C     Other

								  Cc    Control

								  Cf    Format

								  Cn    Unassigned

								  Co    Private use

								  Cs    Surrogate

								.sp

								  L     Letter

								  Ll    Lower case letter

								  Lm    Modifier letter

								  Lo    Other letter

								  Lt    Title case letter

								  Lu    Upper case letter

								.sp

								  M     Mark

								  Mc    Spacing mark

								  Me    Enclosing mark

								  Mn    Non-spacing mark

								.sp

								  N     Number

								  Nd    Decimal number

								  Nl    Letter number

								  No    Other number

								.sp

								  P     Punctuation

								  Pc    Connector punctuation

								  Pd    Dash punctuation

								  Pe    Close punctuation

								  Pf    Final punctuation

								  Pi    Initial punctuation

								  Po    Other punctuation

								  Ps    Open punctuation

								.sp

								  S     Symbol

								  Sc    Currency symbol

								  Sk    Modifier symbol

								  Sm    Mathematical symbol

								  So    Other symbol

								.sp

								  Z     Separator

								  Zl    Line separator

								  Zp    Paragraph separator

								  Zs    Space separator

								.sp

								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".

								.P

								The Cs (Surrogate) property applies only to characters whose code points are in

								the range U+D800 to U+DFFF. These characters are no different 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 validity checking has been turned off (see the

								discussion of PCRE2_NO_UTF_CHECK in the

								.\" HREF

								\fBpcre2api\fP

								.\"

								page).

								.P

								The long synonyms for property names that Perl supports (such as \ep{Letter})

								are not supported by PCRE2, nor is it permitted to prefix any of these

								properties with "Is".

								.P

								No character that is in the Unicode table has the Cn (unassigned) property.

								Instead, this property is assumed for any code point that is not in the

								Unicode table.

								.P

								Specifying caseless matching does not affect these escape sequences. For

								example, \ep{Lu} always matches only upper case letters. This is different from

								the behaviour of current versions of Perl.

								.

								.

								.SS "Binary (yes/no) properties for \ep and \eP"

								.rs

								.sp

								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

								\ep and \eP, along with their abbreviations, by running this command:

								.sp

								  pcre2test -LP

								.sp

								.

								.

								.SS "The Bidi_Class property for \ep and \eP"

								.rs

								.sp

								  \ep{Bidi_Class:<class>}   matches a character with the given class

								  \ep{BC:<class>}           matches a character with the given class

								.sp

								The recognized classes are:

								.sp

								  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

								.sp

								An equals sign may be used instead of a colon. The class names are

								case-insensitive; only the short names listed above are recognized.

								.

								.

								.SS Extended grapheme clusters

								.rs

								.sp

								The \eX escape matches any number of Unicode characters that form an "extended

								grapheme cluster", and treats the sequence as an atomic group

								.\" HTML <a href="#atomicgroup">

								.\" </a>

								(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 various emoji-specific properties. PCRE2 uses only the

								Extended Pictographic property.

								.P

								\eX always matches at least one character. Then it decides whether to add

								additional characters according to the following rules for ending a cluster:

								.P

								1. End at the end of the subject string.

								.P

								2. Do not end between CR and LF; otherwise end after any control character.

								.P

								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 followed only by a T character.

								.P

								4. Do not end before extending characters or spacing marks or the zero-width

								joiner (ZWJ) character. Characters with the "mark" property always have the

								"extend" grapheme breaking property.

								.P

								5. Do not end after prepend characters.

								.P

								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.

								.P

								7. Do not break within emoji flag sequences. That is, do not break between

								regional indicator (RI) characters if there are an odd number of RI characters

								before the break point.

								.P

								8. Otherwise, end the cluster.

								.

								.

								.\" HTML <a name="extraprops"></a>

								.SS PCRE2's additional properties

								.rs

								.sp

								As well as the standard Unicode properties described above, PCRE2 supports four

								more that make it possible to convert traditional escape sequences such as \ew

								and \es 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:

								.sp

								  Xan   Any alphanumeric character

								  Xps   Any POSIX space character

								  Xsp   Any Perl space character

								  Xwd   Any Perl "word" character

								.sp

								Xan matches characters that have either the L (letter) or the N (number)

								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 exclude vertical tab, for Perl

								compatibility, but Perl changed. Xwd matches the same characters as Xan, plus

								those that match Mn (non-spacing mark) or Pc (connector punctuation, which

								includes underscore).

								.P

								There is another non-standard property, Xuc, which matches any character 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 \euHHHH or \eUHHHHHHHH

								where H is a hexadecimal digit. Note that the Xuc property does not match these

								sequences but the characters that they represent.)

								.

								.

								.\" HTML <a name="resetmatchstart"></a>

								.SS "Resetting the match start"

								.rs

								.sp

								In normal use, the escape sequence \eK causes any previously matched characters

								not to be included in the final matched sequence that is returned. For example,

								the pattern:

								.sp

								  foo\eKbar

								.sp

								matches "foobar", but reports that it has matched "bar". \eK does not interact

								with anchoring in any way. The pattern:

								.sp

								  ^foo\eKbar

								.sp

								matches only when the subject begins with "foobar" (in single line mode),

								though it again reports the matched string as "bar". This feature is similar to

								a lookbehind assertion

								.\" HTML <a href="#lookbehind">

								.\" </a>

								(described below),

								.\"

								but the part of the pattern that precedes \eK is not constrained to match a

								limited number of characters, as is required for a lookbehind assertion. The

								use of \eK does not interfere with the setting of

								.\" HTML <a href="#group">

								.\" </a>

								captured substrings.

								.\"

								For example, when the pattern

								.sp

								  (foo)\eKbar

								.sp

								matches "foobar", the first substring is still set to "foo".

								.P

								From version 5.32.0 Perl forbids the use of \eK in lookaround assertions. From

								release 10.38 PCRE2 also forbids this by default. However, the

								PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK option can be used when calling

								\fBpcre2_compile()\fP to re-enable the previous behaviour. When this option is

								set, \eK is acted upon when it occurs inside positive assertions, but is

								ignored in negative assertions. Note that when a pattern such as (?=ab\eK)

								matches, the reported start of the match can be greater than the end of the

								match. Using \eK in a lookbehind assertion at the start of a pattern can also

								lead to odd effects. For example, consider this pattern:

								.sp

								  (?<=\eKfoo)bar

								.sp

								If the subject is "foobar", a call to \fBpcre2_match()\fP 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.

								.

								.

								.\" HTML <a name="smallassertions"></a>

								.SS "Simple assertions"

								.rs

								.sp

								The final use of backslash is for certain simple assertions. An assertion

								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

								.\" HTML <a href="#bigassertions">

								.\" </a>

								below.

								.\"

								The backslashed assertions are:

								.sp

								  \eb     matches at a word boundary

								  \eB     matches when not at a word boundary

								  \eA     matches at the start of the subject

								  \eZ     matches at the end of the subject

								          also matches before a newline at the end of the subject

								  \ez     matches only at the end of the subject

								  \eG     matches at the first matching position in the subject

								.sp

								Inside a character class, \eb 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.

								.P

								A word boundary is a position in the subject string where the current character

								and the previous character do not both match \ew or \eW (i.e. one matches

								\ew and the other matches \eW), or the start or end of the string if the

								first or last character matches \ew, respectively. When PCRE2 is built with

								Unicode support, the meanings of \ew and \eW can be changed by setting the

								PCRE2_UCP option. When this is done, it also affects \eb and \eB. Neither PCRE2

								nor Perl has a separate "start of word" or "end of word" metasequence. However,

								whatever follows \eb normally determines which it is. For example, the fragment

								\eba matches "a" at the start of a word.

								.P

								The \eA, \eZ, and \ez 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 assertions are not affected by the

								PCRE2_NOTBOL or PCRE2_NOTEOL options, which affect only the behaviour of the

								circumflex and dollar metacharacters. However, if the \fIstartoffset\fP

								argument of \fBpcre2_match()\fP is non-zero, indicating that matching is to

								start at a point other than the beginning of the subject, \eA can never match.

								The difference between \eZ and \ez is that \eZ matches before a newline at the

								end of the string as well as at the very end, whereas \ez matches only at the

								end.

								.P

								The \eG assertion is true only when the current matching position is at the

								start point of the matching process, as specified by the \fIstartoffset\fP

								argument of \fBpcre2_match()\fP. It differs from \eA when the value of

								\fIstartoffset\fP is non-zero. By calling \fBpcre2_match()\fP multiple times

								with appropriate arguments, you can mimic Perl's /g option, and it is in this

								kind of implementation where \eG can be useful.

								.P

								Note, however, that PCRE2's implementation of \eG, 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.

								.P

								If all the alternatives of a pattern begin with \eG, the expression is anchored

								to the starting match position, and the "anchored" flag is set in the compiled

								regular expression.

								.

								.

								.SH "CIRCUMFLEX AND DOLLAR"

								.rs

								.sp

								The circumflex and dollar metacharacters are zero-width assertions. That is,

								they test for a particular condition being true without consuming any

								characters from the subject string. These two metacharacters are concerned with

								matching the starts and ends of lines. If the newline 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.

								.P

								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 \fIstartoffset\fP argument of

								\fBpcre2_match()\fP is non-zero, or if PCRE2_NOTBOL is set, circumflex can

								never match if the PCRE2_MULTILINE option is unset. Inside a character class,

								circumflex has an entirely different meaning

								.\" HTML <a href="#characterclass">

								.\" </a>

								(see below).

								.\"

								.P

								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 subject, it is said to be an

								"anchored" pattern. (There are also other constructs that can cause a pattern

								to be anchored.)

								.P

								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 before a newline at

								the end of the string (by default), unless PCRE2_NOTEOL is set. Note, however,

								that it does not actually match the newline. 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 character class.

								.P

								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 \eZ assertion.

								.P

								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.

								.P

								For example, the pattern /^abc$/ matches the subject string "def\enabc" (where

								\en 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 \fIstartoffset\fP argument of \fBpcre2_match()\fP is non-zero. The

								PCRE2_DOLLAR_ENDONLY option is ignored if PCRE2_MULTILINE is set.

								.P

								When the newline convention (see

								.\" HTML <a href="#newlines">

								.\" </a>

								"Newline conventions"

								.\"

								below) recognizes the two-character sequence CRLF as a newline, this is

								preferred, even if the single characters CR and LF are also recognized as

								newlines. For example, if the newline convention is "any", a multiline mode

								circumflex matches before "xyz" in the string "abc\er\enxyz" 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.)

								.P

								Note that the sequences \eA, \eZ, and \ez can be used to match the start and

								end of the subject in both modes, and if all branches of a pattern start with

								\eA it is always anchored, whether or not PCRE2_MULTILINE is set.

								.

								.

								.\" HTML <a name="fullstopdot"></a>

								.SH "FULL STOP (PERIOD, DOT) AND \eN"

								.rs

								.sp

								Outside a character class, a dot in the pattern matches any one character in

								the subject string except (by default) a character that signifies the end of a

								line. One or more characters may be specified as line terminators (see

								.\" HTML <a href="#newlines">

								.\" </a>

								"Newline conventions"

								.\"

								above).

								.P

								Dot never matches a single line-ending character. When the two-character

								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.

								.P

								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 subject string, it takes

								two dots to match it.

								.P

								The handling of dot is entirely independent of the handling of circumflex and

								dollar, the only relationship being that they both involve newlines. Dot has no

								special meaning in a character class.

								.P

								The escape sequence \eN 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.

								.P

								When \eN is followed by an opening brace it has a different meaning. See the

								section entitled

								.\" HTML <a href="digitsafterbackslash">

								.\" </a>

								"Non-printing characters"

								.\"

								above for details. Perl also uses \eN{name} to specify characters by Unicode

								name; PCRE2 does not support this.

								.

								.

								.SH "MATCHING A SINGLE CODE UNIT"

								.rs

								.sp

								Outside a character class, the escape sequence \eC 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, \eC 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 usefully be used.

								.P

								Because \eC breaks up characters into individual code units, matching one unit

								with \eC in UTF-8 or UTF-16 mode means that the rest of the string may start

								with a malformed UTF character. This has undefined results, because PCRE2

								assumes that it is matching character by character in a valid UTF string (by

								default it checks the subject string's validity at the start of processing

								unless the PCRE2_NO_UTF_CHECK or PCRE2_MATCH_INVALID_UTF option is used).

								.P

								An application can lock out the use of \eC by setting the

								PCRE2_NEVER_BACKSLASH_C option when compiling a pattern. It is also possible to

								build PCRE2 with the use of \eC permanently disabled.

								.P

								PCRE2 does not allow \eC to appear in lookbehind assertions

								.\" HTML <a href="#lookbehind">

								.\" </a>

								(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

								\fBpcre2_dfa_match()\fP nor the JIT optimizer support \eC 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.

								.P

								In the 32-bit library, however, \eC is always supported (when not explicitly

								locked out) because it always matches a single code unit, whether or not UTF-32

								is specified.

								.P

								In general, the \eC escape sequence is best avoided. However, one way of using

								it that avoids the problem of malformed UTF-8 or UTF-16 characters 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):

								.sp

								  (?| (?=[\ex00-\ex7f])(\eC) |

								      (?=[\ex80-\ex{7ff}])(\eC)(\eC) |

								      (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |

								      (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))

								.sp

								In this example, a group that starts with (?| resets the capturing parentheses

								numbers in each alternative (see

								.\" HTML <a href="#dupgroupnumber">

								.\" </a>

								"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, respectively. The

								character's individual bytes are then captured by the appropriate number of

								\eC groups.

								.

								.

								.\" HTML <a name="characterclass"></a>

								.SH "SQUARE BRACKETS AND CHARACTER CLASSES"

								.rs

								.sp

								An opening square bracket introduces a character class, terminated by a closing

								square bracket. A closing square bracket on its own is not special 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.

								.P

								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.

								.P

								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 consumes a character from the subject

								string, and therefore it fails if the current pointer is at the end of the

								string.

								.P

								Characters in a class may be specified by their code points using \eo, \ex, or

								\eN{U+hh..} in the usual way. When caseless matching is set, any letters in a

								class represent both their upper case and lower case versions, 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 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.

								.P

								Characters that might indicate line breaks are never treated in any special way

								when matching character classes, whatever line-ending sequence 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.

								.P

								The generic character type escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es,

								\eS, \ev, \eV, \ew, and \eW may appear in a character class, and add the

								characters that they match to the class. For example, [\edABCDEF] matches any

								hexadecimal digit. In UTF modes, the PCRE2_UCP option affects the meanings of

								\ed, \es, \ew and their upper case partners, just as it does when they appear

								outside a character class, as described in the section entitled

								.\" HTML <a href="#genericchartypes">

								.\" </a>

								"Generic character types"

								.\"

								above. The escape sequence \eb has a different meaning inside a character

								class; it matches the backspace character. The sequences \eB, \eR, and \eX are

								not special inside a character class. Like any other unrecognized escape

								sequences, they cause an error. The same is true for \eN when not followed by

								an opening brace.

								.P

								The minus (hyphen) character can be used to specify a range of characters in a

								character class. For example, [d-m] matches any letter between 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.

								.P

								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 \ed or \eH.

								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.

								.P

								It is not possible to have the literal character "]" as the end character 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-\e]46] is interpreted 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.

								.P

								Ranges normally include all code points between the start and end characters,

								inclusive. They can also be used for code points specified numerically, for

								example [\e000-\e037]. 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). However, ranges such as [\ex{d7ff}-\ex{e000}], which include the

								surrogates, are always permitted.

								.P

								There is a special case in EBCDIC environments for ranges whose end points are

								both specified as literal letters in the same case. For compatibility 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, [\ex88-\ex92] or [h-\ex92], all code points

								are included.

								.P

								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

								[][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character

								tables for a French locale are in use, [\exc8-\excb] matches accented E

								characters in both cases.

								.P

								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 [^\eW_] matches any letter or digit, but not underscore,

								whereas [\ew] 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 ...".

								.P

								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-alphanumeric characters does no harm.

								.

								.

								.SH "POSIX CHARACTER CLASSES"

								.rs

								.sp

								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,

								.sp

								  [01[:alpha:]%]

								.sp

								matches "0", "1", any alphabetic character, or "%". The supported class names

								are:

								.sp

								  alnum    letters and digits

								  alpha    letters

								  ascii    character codes 0 - 127

								  blank    space or tab only

								  cntrl    control characters

								  digit    decimal digits (same as \ed)

								  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 \es from PCRE2 8.34)

								  upper    upper case letters

								  word     "word" characters (same as \ew)

								  xdigit   hexadecimal digits

								.sp

								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

								\es match the same set of characters, as do "word" and \ew.

								.P

								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,

								.sp

								  [12[:^digit:]]

								.sp

								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.

								.P

								By default, characters with values greater than 127 do not match any of the

								POSIX character classes, although this may be different for characters 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:

								.sp

								  [:alnum:]  becomes  \ep{Xan}

								  [:alpha:]  becomes  \ep{L}

								  [:blank:]  becomes  \eh

								  [:cntrl:]  becomes  \ep{Cc}

								  [:digit:]  becomes  \ep{Nd}

								  [:lower:]  becomes  \ep{Ll}

								  [:space:]  becomes  \ep{Xps}

								  [:upper:]  becomes  \ep{Lu}

								  [:word:]   becomes  \ep{Xwd}

								.sp

								Negated versions, such as [:^alpha:] use \eP instead of \ep. Four other POSIX

								classes are handled specially in UCP mode:

								.TP 10

								[:graph:]

								This matches characters that have glyphs that mark the page when printed. In

								Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf

								properties, except for:

								.sp

								  U+061C           Arabic Letter Mark

								  U+180E           Mongolian Vowel Separator

								  U+2066 - U+2069  Various "isolate"s

								.sp

								.TP 10

								[:print:]

								This matches the same characters as [:graph:] plus space characters that are

								not controls, that is, characters with the Zs property.

								.TP 10

								[:punct:]

								This matches all characters that have the Unicode P (punctuation) property,

								plus those characters with code points less than 256 that have the S (Symbol)

								property.

								.TP 10

								[: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.

								.P

								The other POSIX classes are unchanged by PCRE2_UCP, and match only characters

								with code points less than 256.

								.P

								There are two options that can be used to restrict the POSIX classes to ASCII

								characters when PCRE2_UCP is set. The option PCRE2_EXTRA_ASCII_DIGIT affects

								just [:digit:] and [:xdigit:]. Within a pattern, this can be set and unset by

								(?aT) and (?-aT). The PCRE2_EXTRA_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.

								.

								.

								.SH "COMPATIBILITY FEATURE FOR WORD BOUNDARIES"

								.rs

								.sp

								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:

								.sp

								  [[:<:]]  is converted to  \eb(?=\ew)

								  [[:>:]]  is converted to  \eb(?<=\ew)

								.sp

								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 \eb matches

								at the start and the end of a word (see

								.\" HTML <a href="#smallassertions">

								.\" </a>

								"Simple assertions"

								.\"

								above), and in a Perl-style pattern the preceding or following character

								normally shows which is wanted, without the need for the assertions that are

								used above in order to give exactly the POSIX behaviour. Note also that the

								PCRE2_UCP option changes the meaning of \ew (and therefore \eb) by default, so

								it also affects these POSIX sequences.

								.

								.

								.SH "VERTICAL BAR"

								.rs

								.sp

								Vertical bar characters are used to separate alternative patterns. For example,

								the pattern

								.sp

								  gilbert|sullivan

								.sp

								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

								.\" HTML <a href="#group">

								.\" </a>

								(defined below),

								.\"

								"succeeds" means matching the rest of the main pattern as well as the

								alternative in the group.

								.

								.

								.\" HTML <a name="internaloptions"></a>

								.SH "INTERNAL OPTION SETTING"

								.rs

								.sp

								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

								.\" HREF

								\fBpcre2api\fP

								.\"

								documentation. The option letters are:

								.sp

								  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

								.sp

								For example, (?im) sets caseless, multiline matching. It is also possible to

								unset these options by preceding the relevant letters with a hyphen, for

								example (?-im). The two "extended" options are not independent; unsetting

								either one cancels the effects of both of them.

								.P

								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. Needless to say, it has no effect.

								.P

								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.

								.P

								Some PCRE2-specific options can be changed by the same mechanism using these

								pairs or individual letters:

								.sp

								  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

								.sp

								However, except for 'r', these are not unset by (?^), which is equivalent to

								(?-imnrsx). If 'a' is not followed by any of the upper case letters shown

								above, it sets (or unsets) all the ASCII options.

								.P

								PCRE2_EXTRA_ASCII_DIGIT has no additional effect when PCRE2_EXTRA_ASCII_POSIX

								is set, but including it in (?aP) means that (?-aP) suppresses all ASCII

								restrictions for POSIX classes.

								.P

								When one of these option changes occurs at top level (that is, not inside 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,

								.sp

								  (a(?i)b)c

								.sp

								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,

								.sp

								  (a(?i)b|c)

								.sp

								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.

								.P

								As a convenient shorthand, if any option settings are required at the start of

								a non-capturing group (see the next section), the option letters may

								appear between the "?" and the ":". Thus the two patterns

								.sp

								  (?i:saturday|sunday)

								  (?:(?i)saturday|sunday)

								.sp

								match exactly the same set of strings.

								.P

								\fBNote:\fP There are other PCRE2-specific options, applying to the whole

								pattern, which can be set by the application when the compiling function 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

								.\" HTML <a href="#newlineseq">

								.\" </a>

								"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, respectively. 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.

								.

								.

								.\" HTML <a name="group"></a>

								.SH GROUPS

								.rs

								.sp

								Groups are delimited by parentheses (round brackets), which can be nested.

								Turning part of a pattern into a group does two things:

								.sp

								1. It localizes a set of alternatives. For example, the pattern

								.sp

								  cat(aract|erpillar|)

								.sp

								matches "cataract", "caterpillar", or "cat". Without the parentheses, it would

								match "cataract", "erpillar" or an empty string.

								.sp

								2. It creates a "capture group". This means that, when the whole pattern

								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.)

								.P

								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

								.sp

								  the ((red|white) (king|queen))

								.sp

								the captured substrings are "red king", "red", and "king", and are numbered 1,

								2, and 3, respectively.

								.P

								The fact that plain parentheses fulfil two functions is not always helpful.

								There are often times when grouping is required without capturing. 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

								.sp

								  the ((?:red|white) (king|queen))

								.sp

								the captured substrings are "white queen" and "queen", and are numbered 1 and

								2. The maximum number of capture groups is 65535.

								.P

								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

								.sp

								  (?i:saturday|sunday)

								  (?:(?i)saturday|sunday)

								.sp

								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 subsequent branches, so

								the above patterns match "SUNDAY" as well as "Saturday".

								.

								.

								.\" HTML <a name="dupgroupnumber"></a>

								.SH "DUPLICATE GROUP NUMBERS"

								.rs

								.sp

								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:

								.sp

								  (?|(Sat)ur|(Sun))day

								.sp

								Because the two alternatives are inside a (?| group, both sets of capturing

								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, parentheses 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 following example is taken from the Perl documentation. The

								numbers underneath show in which buffer the captured content will be stored.

								.sp

								  # before  ---------------branch-reset----------- after

								  / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x

								  # 1            2         2  3        2     3     4

								.sp

								A backreference to a capture group uses the most recent value that is set for

								the group. The following pattern matches "abcabc" or "defdef":

								.sp

								  /(?|(abc)|(def))\e1/

								.sp

								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":

								.sp

								  /(?|(abc)|(def))(?1)/

								.sp

								A relative reference such as (?-1) is no different: it is just a convenient way

								of computing an absolute group number.

								.P

								If a

								.\" HTML <a href="#conditions">

								.\" </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.

								.P

								An alternative approach to using this "branch reset" feature is to use

								duplicate named groups, as described in the next section.

								.

								.

								.SH "NAMED CAPTURE GROUPS"

								.rs

								.sp

								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 earlier, and PCRE1 introduced it at release 4.0,

								using the Python syntax. PCRE2 supports both the Perl and the Python syntax.

								.P

								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:

								.sp

								  ^[_A-Za-z][_A-Za-z0-9]*\ez   when PCRE2_UTF is not set

								  ^[_\ep{L}][_\ep{L}\ep{Nd}]*\ez  when PCRE2_UTF is set

								.sp

								References to capture groups from other parts of the pattern, such as

								.\" HTML <a href="#backreferences">

								.\" </a>

								backreferences,

								.\"

								.\" HTML <a href="#recursion">

								.\" </a>

								recursion,

								.\"

								and

								.\" HTML <a href="#conditions">

								.\" </a>

								conditions,

								.\"

								can all be made by name as well as by number.

								.P

								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.

								.P

								\fBWarning:\fP When more than one capture group has the same number, as

								described 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:

								.sp

								  (?|(?<AA>aa)|(?<BB>bb))

								.sp

								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").

								.P

								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 provokes a

								compile-time error. However, there is still scope for confusion. Consider this

								pattern:

								.sp

								  (?|(?<AA>aa)|(bb))

								.sp

								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.

								.P

								By default, a name must be unique within a pattern, except that duplicate names

								are permitted for groups with the same number, for example:

								.sp

								  (?|(?<AA>aa)|(?<AA>bb))

								.sp

								The duplicate name constraint can be disabled by setting the PCRE2_DUPNAMES

								option at compile time, or by the use of (?J) within the pattern, as described

								in the section entitled

								.\" HTML <a href="#internaloptions">

								.\" </a>

								"Internal Option Setting"

								.\"

								above.

								.P

								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:

								.sp

								  (?J)

								  (?<DN>Mon|Fri|Sun)(?:day)?|

								  (?<DN>Tue)(?:sday)?|

								  (?<DN>Wed)(?:nesday)?|

								  (?<DN>Thu)(?:rsday)?|

								  (?<DN>Sat)(?:urday)?

								.sp

								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.)

								.P

								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":

								.sp

								  (?J)(?:(?<n>foo)|(?<n>bar))\ek<n>

								.sp

								.P

								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.

								.P

								If you use a named reference in a condition

								test (see the

								.\"

								.\" HTML <a href="#conditions">

								.\" </a>

								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 details of the interfaces for handling named

								capture groups, see the

								.\" HREF

								\fBpcre2api\fP

								.\"

								documentation.

								.

								.

								.SH REPETITION

								.rs

								.sp

								Repetition is specified by quantifiers, which may follow any one of these

								items:

								.sp

								  a literal data character

								  the dot metacharacter

								  the \eC escape sequence

								  the \eR escape sequence

								  the \eX 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)

								.sp

								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,

								.sp

								  z{2,4}

								.sp

								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

								.sp

								  [aeiou]{3,}

								.sp

								matches at least 3 successive vowels, but may match many more, whereas

								.sp

								  \ed{8}

								.sp

								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.

								.sp

								  X{,4} is interpreted as X{0,4}

								.sp

								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.

								.P

								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 particular, this

								means that {,} is a literal string of three characters.

								.P

								Note that not every opening brace is potentially the start of a quantifier

								because braces are used in other items such as \eN{U+345} or \ek{name}.

								.P

								In UTF modes, quantifiers apply to characters rather than to individual code

								units. Thus, for example, \ex{100}{2} matches two characters, each of

								which is represented by a two-byte sequence in a UTF-8 string. Similarly,

								\eX{3} matches three Unicode extended grapheme clusters, each of which may be

								several code units long (and they may be of different lengths).

								.P

								The quantifier {0} is permitted, causing the expression to behave as if the

								previous item and the quantifier were not present. This may be useful for

								capture groups that are referenced as

								.\" HTML <a href="#groupsassubroutines">

								.\" </a>

								subroutines

								.\"

								from elsewhere in the pattern (but see also the section entitled

								.\" HTML <a href="#subdefine">

								.\" </a>

								"Defining capture groups for use by reference only"

								.\"

								below). Except for parenthesized groups, items that have a {0} quantifier are

								omitted from the compiled pattern.

								.P

								For convenience, the three most common quantifiers have single-character

								abbreviations:

								.sp

								  *    is equivalent to {0,}

								  +    is equivalent to {1,}

								  ?    is equivalent to {0,1}

								.sp

								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:

								.sp

								  (a?)*

								.sp

								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 subsequent item fails to match.

								.P

								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

								.sp

								  /\e*.*\e*/

								.sp

								to the string

								.sp

								  /* first comment */  not comment  /* second comment */

								.sp

								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

								.sp

								  /\e*.*?\e*/

								.sp

								does the right thing with C comments. The meaning of the various quantifiers 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

								.sp

								  \ed??\ed

								.sp

								which matches one digit by preference, but can match two if that is the only

								way the rest of the pattern matches.

								.P

								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.

								.P

								When a parenthesized group is quantified with a minimum repeat count that

								is greater than 1 or with a limited maximum, more memory is required for the

								compiled pattern, in proportion to the size of the minimum or maximum.

								.P

								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 newlines, 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 after the first. PCRE2 normally treats such a

								pattern as though it were preceded by \eA.

								.P

								In cases where it is known that the subject string contains no newlines, it is

								worth setting PCRE2_DOTALL in order to obtain this optimization, or

								alternatively, using ^ to indicate anchoring explicitly.

								.P

								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:

								.sp

								  (.*)abc\e1

								.sp

								If the subject is "xyz123abc123" the match point is the fourth character. For

								this reason, such a pattern is not implicitly anchored.

								.P

								Another case where implicit anchoring is not applied is when the leading .* is

								inside an atomic group. Once again, a match at the start may fail where a later

								one succeeds. Consider this pattern:

								.sp

								  (?>.*?a)b

								.sp

								It matches "ab" in the subject "aab". The use of the backtracking control verbs

								(*PRUNE) and (*SKIP) also disable this optimization, and there is an option,

								PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly.

								.P

								When a capture group is repeated, the value captured is the substring that

								matched the final iteration. For example, after

								.sp

								  (tweedle[dume]{3}\es*)+

								.sp

								has matched "tweedledum tweedledee" the value of the captured substring is

								"tweedledee". However, if there are nested capture groups, the corresponding

								captured values may have been set in previous iterations. For example, after

								.sp

								  (a|(b))+

								.sp

								matches "aba" the value of the second captured substring is "b".

								.

								.

								.\" HTML <a name="atomicgroup"></a>

								.SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"

								.rs

								.sp

								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.

								.P

								Consider, for example, the pattern \ed+foo when applied to the subject line

								.sp

								  123456bar

								.sp

								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 \ed+

								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.

								.P

								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:

								.sp

								  (?>\ed+)foo

								.sp

								Perl 5.28 introduced an experimental alphabetic form starting with (* which may

								be easier to remember:

								.sp

								  (*atomic:\ed+)foo

								.sp

								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.

								.P

								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.

								.P

								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 \ed+ and \ed+? are prepared to adjust the number of digits they

								match in order to make the rest of the pattern match, (?>\ed+) can only match

								an entire sequence of digits.

								.P

								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 simpler

								notation, called a "possessive quantifier" can be used. This consists of an

								additional + character following a quantifier. Using this notation, the

								previous example can be rewritten as

								.sp

								  \ed++foo

								.sp

								Note that a possessive quantifier can be used with an entire group, for

								example:

								.sp

								  (abc|xyz){2,3}+

								.sp

								Possessive quantifiers are always greedy; the setting of the PCRE2_UNGREEDY

								option is ignored. They are a convenient notation for the simpler 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.

								.P

								The possessive quantifier syntax is an extension to the Perl 5.8 syntax.

								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.

								.P

								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_AUTOPOSSESS option, or starting

								the pattern with (*NO_AUTO_POSSESS).

								.P

								When a pattern contains an unlimited repeat inside a group that can itself 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

								.sp

								  (\eD+|<\ed+>)*[!?]

								.sp

								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

								.sp

								  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

								.sp

								it takes a long time before reporting failure. This is because the string can

								be divided between the internal \eD+ repeat and the external * repeat in a

								large number of ways, and all have to be tried. (The example 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:

								.sp

								  ((?>\eD+)|<\ed+>)*[!?]

								.sp

								sequences of non-digits cannot be broken, and failure happens quickly.

								.

								.

								.\" HTML <a name="backreferences"></a>

								.SH "BACKREFERENCES"

								.rs

								.sp

								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.

								.P

								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 reference 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.

								.P

								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 \e50 is

								interpreted as a character defined in octal. See the subsection entitled

								"Non-printing characters"

								.\" HTML <a href="#digitsafterbackslash">

								.\" </a>

								above

								.\"

								for further details of the handling of digits following a backslash. Other

								forms of backreferencing do not suffer from this restriction. In particular,

								there is no problem when named capture groups are used (see below).

								.P

								Another way of avoiding the ambiguity inherent in the use of digits following a

								backslash is to use the \eg escape sequence. This escape must be followed by a

								signed or unsigned number, optionally enclosed in braces. These examples are

								all identical:

								.sp

								  (ring), \e1

								  (ring), \eg1

								  (ring), \eg{1}

								.sp

								An unsigned number specifies an absolute reference without the ambiguity 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:

								.sp

								  (abc(def)ghi)\eg{-1}

								.sp

								The sequence \eg{-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 \e2, and \eg{-2} would

								be equivalent to \e1. Note that if this construct is inside a capture group,

								that group is included in the count, so in this example \eg{-2} also refers to

								group 1:

								.sp

								  (A)(\eg{-2}B)

								.sp

								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.

								.P

								The sequence \eg{+1} is a reference to the next capture group that is started

								after this item, and \eg{+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.

								.P

								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

								.\" HTML <a href="#groupsassubroutines">

								.\" </a>

								"Groups as subroutines"

								.\"

								below for a way of doing that). So the pattern

								.sp

								  (sens|respons)e and \e1ibility

								.sp

								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 example,

								.sp

								  ((?i)rah)\es+\e1

								.sp

								matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original

								capture group is matched caselessly.

								.P

								There are several different ways of writing backreferences to named capture

								groups. The .NET syntax is \ek{name}, the Python syntax is (?=name), and the

								original Perl syntax is \ek<name> or \ek'name'. All of these are now supported

								by both Perl and PCRE2. Perl 5.10's unified backreference syntax, in which \eg

								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:

								.sp

								  (?<p1>(?i)rah)\es+\ek<p1>

								  (?'p1'(?i)rah)\es+\ek{p1}

								  (?P<p1>(?i)rah)\es+(?P=p1)

								  (?<p1>(?i)rah)\es+\eg{p1}

								.sp

								A capture group that is referenced by name may appear in the pattern before or

								after the reference.

								.P

								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

								.sp

								  (a|(bc))\e2

								.sp

								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 backreference to an

								unset value matches an empty string.

								.P

								Because there may be many capture groups in a pattern, all digits following a

								backslash are taken as part of a potential backreference number. 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 \eg{} syntax or an empty comment (see

								.\" HTML <a href="#comments">

								.\" </a>

								"Comments"

								.\"

								below) can be used.

								.

								.

								.SS "Recursive backreferences"

								.rs

								.sp

								A backreference that occurs inside the group to which it refers fails when the

								group is first used, so, for example, (a\e1) never matches. However, such

								references can be useful inside repeated groups. For example, the pattern

								.sp

								  (a|b\e1)+

								.sp

								matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of

								the group, the backreference matches the character string corresponding 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 example above, or by a quantifier with a minimum

								of zero.

								.P

								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

								.\" HTML <a href="#atomicgroup">

								.\" </a>

								atomic group.

								.\"

								This restriction no longer applies, and backtracking into such groups can occur

								as normal.

								.

								.

								.\" HTML <a name="bigassertions"></a>

								.SH ASSERTIONS

								.rs

								.sp

								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 \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described

								.\" HTML <a href="#smallassertions">

								.\" </a>

								above.

								.\"

								.P

								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 assertion 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 continues after it, but with the matching position

								in the subject string reset to what it was before the assertion was processed.

								.P

								The Perl-compatible lookaround assertions are atomic. If an assertion is true,

								but there is a subsequent matching failure, there is no backtracking into the

								assertion. However, there are some cases where non-atomic assertions can be

								useful. PCRE2 has some support for these, described in the section entitled

								.\" HTML <a href="#nonatomicassertions">

								.\" </a>

								"Non-atomic assertions"

								.\"

								below, but they are not Perl-compatible.

								.P

								A lookaround assertion may appear as the condition in a

								.\" HTML <a href="#conditions">

								.\" </a>

								conditional group

								.\"

								(see below). In this case, the result of matching the assertion determines

								which branch of the condition is followed.

								.P

								Assertion groups are not capture groups. If an assertion contains capture

								groups within it, these are counted for the purposes of numbering the capture

								groups in the whole pattern. Within each branch of an assertion, locally

								captured substrings may be referenced in the usual way. For example, a sequence

								such as (.)\eg{-1} can be used to check that two adjacent characters are the

								same.

								.P

								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 contains a matching branch, what happens

								depends on the type of assertion.

								.P

								For a positive assertion, internally captured substrings in the successful

								branch are retained, and matching continues with the next pattern 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

								.\" HTML <a href="#conditions">

								.\" </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.

								.P

								Most assertion groups may be repeated; though it makes no sense to assert the

								same thing several times, the side effect of capturing in positive 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}.

								.

								.

								.SS "Alphabetic assertion names"

								.rs

								.sp

								Traditionally, symbolic sequences such as (?= and (?<= have been used to

								specify lookaround assertions. Perl 5.28 introduced some experimental

								alphabetic alternatives which might be easier to remember. They all start with

								(* instead of (? and must be written using lower case letters. PCRE2 supports

								the following synonyms:

								.sp

								  (*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 (?<!

								.sp

								For example, (*pla:foo) is the same assertion as (?=foo). In the following

								sections, the various assertions are described using the original symbolic

								forms.

								.

								.

								.SS "Lookahead assertions"

								.rs

								.sp

								Lookahead assertions start with (?= for positive assertions and (?! for

								negative assertions. For example,

								.sp

								  \ew+(?=;)

								.sp

								matches a word followed by a semicolon, but does not include the semicolon in

								the match, and

								.sp

								  foo(?!bar)

								.sp

								matches any occurrence of "foo" that is not followed by "bar". Note that the

								apparently similar pattern

								.sp

								  (?!foo)bar

								.sp

								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.

								.P

								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 always 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 (?!).

								.

								.

								.\" HTML <a name="lookbehind"></a>

								.SS "Lookbehind assertions"

								.rs

								.sp

								Lookbehind assertions start with (?<= for positive assertions and (?<! for

								negative assertions. For example,

								.sp

								  (?<!foo)bar

								.sp

								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:

								.P

								If every top-level alternative matches a fixed length, for example

								.sp

								  (?<=colour|color)

								.sp

								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 \fBpcre2_dfa_match()\fP.

								.P

								In PCRE2 10.43 and later, \fBpcre2_match()\fP supports lookbehind assertions in

								which one or more top-level alternatives can match more than one string length,

								for example

								.sp

								  (?<=colou?r)

								.sp

								The maximum matching length for any branch of the lookbehind is limited to a

								value set by the calling program (default 255 characters). Unlimited repetition

								(for example \ed*) is not supported. In some cases, the escape sequence \eK

								.\" HTML <a href="#resetmatchstart">

								.\" </a>

								(see above)

								.\"

								can be used instead of a lookbehind assertion at the start of a pattern to get

								round the length limit restriction.

								.P

								In UTF-8 and UTF-16 modes, PCRE2 does not allow the \eC 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

								\eX and \eR escapes, which can match different numbers of code units, are never

								permitted in lookbehinds.

								.P

								.\" HTML <a href="#groupsassubroutines">

								.\" </a>

								"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,

								.\" HTML <a href="#recursion">

								.\" </a>

								recursion,

								.\"

								that is, a "subroutine" call into a group that is already active,

								is not supported.

								.P

								PCRE2 supports backreferences in lookbehinds, but only if certain conditions

								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 substring. The following pattern

								matches words containing at least two characters that begin and end with the

								same character:

								.sp

								   \eb(\ew)\ew++(?<=\e1)

								.P

								Possessive quantifiers can be used in conjunction with lookbehind assertions to

								specify efficient matching at the end of subject strings. Consider a simple

								pattern such as

								.sp

								  abcd$

								.sp

								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 subject and then see if

								what follows matches the rest of the pattern. If the pattern is specified as

								.sp

								  ^.*abcd$

								.sp

								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

								.sp

								  ^.*+(?<=abcd)

								.sp

								there can be no backtracking for the .*+ item because of the possessive

								quantifier; it can match only the entire string. The subsequent lookbehind

								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.

								.

								.

								.SS "Using multiple assertions"

								.rs

								.sp

								Several assertions (of any sort) may occur in succession. For example,

								.sp

								  (?<=\ed{3})(?<!999)foo

								.sp

								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 \fInot\fP match "foo" preceded by six characters, the first

								of which are digits and the last three of which are not "999". For example, it

								doesn't match "123abcfoo". A pattern to do that is

								.sp

								  (?<=\ed{3}...)(?<!999)foo

								.sp

								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".

								.P

								Assertions can be nested in any combination. For example,

								.sp

								  (?<=(?<!foo)bar)baz

								.sp

								matches an occurrence of "baz" that is preceded by "bar" which in turn is not

								preceded by "foo", while

								.sp

								  (?<=\ed{3}(?!999)...)foo

								.sp

								is another pattern that matches "foo" preceded by three digits and any three

								characters that are not "999".

								.

								.

								.\" HTML <a name="nonatomicassertions"></a>

								.SH "NON-ATOMIC ASSERTIONS"

								.rs

								.sp

								Traditional lookaround assertions are atomic. That is, if an assertion is true,

								but there is a subsequent matching failure, there is no backtracking into the

								assertion. However, there are some cases where non-atomic positive assertions

								can be useful. PCRE2 provides these using the following syntax:

								.sp

								  (*non_atomic_positive_lookahead:  or (*napla: or (?*

								  (*non_atomic_positive_lookbehind: or (*naplb: or (?<*

								.sp

								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:

								.sp

								  ^(?x)(*napla: .* \eb(\ew++)) (?> .*? \eb\e1\eb ){2}

								.sp

								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 readability) 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.

								.P

								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 occur 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.

								.P

								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.

								.P

								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.

								.P

								There is one exception to backtracking into a non-atomic assertion. If an

								(*ACCEPT) control verb is triggered, the assertion succeeds atomically. That

								is, a subsequent match failure cannot backtrack into the assertion.

								.P

								Non-atomic assertions are not supported by the alternative matching function

								\fBpcre2_dfa_match()\fP. 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

								.\" HTML <a href="#conditions">

								.\" </a>

								conditional groups

								.\"

								(see below) must be atomic.

								.

								.

								.SH "SCRIPT RUNS"

								.rs

								.sp

								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

								.\" HTML <a href="pcre2unicode.html#scriptruns">

								.\" </a>

								"Script Runs"

								.\"

								in the

								.\" HREF

								\fBpcre2unicode\fP

								.\"

								documentation.

								.P

								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 occurs. Script runs can be

								used to detect spoofing attacks using characters that look the same, but are

								from different scripts. The string "paypal.com" is an infamous example, where

								the letters could be a mixture of Latin and Cyrillic. This pattern ensures that

								the matched characters in a sequence of non-spaces that follow white space are

								a script run:

								.sp

								  \es+(*sr:\eS+)

								.sp

								To be sure that they are all from the Latin script (for example), a lookahead

								can be used:

								.sp

								  \es+(?=\ep{Latin})(*sr:\eS+)

								.sp

								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:

								.sp

								  \es+(?=[0-9_.]*\ep{Latin})(*sr:\eS+)

								.sp

								.P

								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:

								.sp

								  (*asr:...) is the same as (*sr:(?>...))

								.sp

								Note that the atomic group is inside the script run. Putting it outside would

								not prevent backtracking into the script run pattern.

								.P

								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 constructs is

								encountered. Script runs are not supported by the alternate matching function,

								\fBpcre2_dfa_match()\fP because they use the same mechanism as capturing

								parentheses.

								.P

								\fBWarning:\fP The (*ACCEPT) control verb

								.\" HTML <a href="#acceptverb">

								.\" </a>

								(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.

								.

								.

								.\" HTML <a name="conditions"></a>

								.SH "CONDITIONAL GROUPS"

								.rs

								.sp

								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:

								.sp

								  (?(condition)yes-pattern)

								  (?(condition)yes-pattern|no-pattern)

								.sp

								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 alternatives in the

								group, a compile-time error occurs. Each of the two alternatives may itself

								contain nested groups of any form, including conditional 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:

								.sp

								  (?(1) (A|B|C) | (D | (?(2)E|F) | E) )

								.sp

								.P

								There are five kinds of condition: references to capture groups, references to

								recursion, two pseudo-conditions called DEFINE and VERSION, and assertions.

								.

								.

								.SS "Checking for a used capture group by number"

								.rs

								.sp

								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

								.\"

								.\" HTML <a href="#recursion">

								.\" </a>

								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 refer 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.

								.P

								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:

								.sp

								  ( \e( )?    [^()]+    (?(1) \e) )

								.sp

								The first part matches an optional opening parenthesis, and if that

								character is present, sets it as the first captured substring. The second 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.

								.P

								If you were embedding this pattern in a larger one, you could use a relative

								reference:

								.sp

								  ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...

								.sp

								This makes the fragment independent of the parentheses in the larger pattern.

								.

								.

								.SS "Checking for a used capture group by name"

								.rs

								.sp

								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 section). Rewriting the above example

								to use a named group gives this:

								.sp

								  (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )

								.sp

								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.

								.

								.

								.SS "Checking for pattern recursion"

								.rs

								.sp

								"Recursion" in this sense refers to any subroutine-like call from one part of

								the pattern to another, whether or not it is actually recursive. See the

								sections entitled

								.\" HTML <a href="#recursion">

								.\" </a>

								"Recursive patterns"

								.\"

								and

								.\" HTML <a href="#groupsassubroutines">

								.\" </a>

								"Groups as subroutines"

								.\"

								below for details of recursion and subroutine calls.

								.P

								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 recursion 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:

								.sp

								  ((?(R1)a+|(?1)b))

								.sp

								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 section above, instead

								of testing for recursion. For example, creating a group with the name R1 by

								adding (?<R1>) to the above pattern completely changes its meaning.

								.P

								If a name preceded by ampersand follows the letter R, for example:

								.sp

								  (?(R&name)...)

								.sp

								the condition is true if the most recent recursion is into a group of that name

								(which must exist within the pattern).

								.P

								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.

								.P

								At "top level", all these recursion test conditions are false.

								.

								.

								.\" HTML <a name="subdefine"></a>

								.SS "Defining capture groups for use by reference only"

								.rs

								.sp

								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 always 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 referenced from elsewhere. (The use of

								.\" HTML <a href="#groupsassubroutines">

								.\" </a>

								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):

								.sp

								  (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )

								  \eb (?&byte) (\e.(?&byte)){3} \eb

								.sp

								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, insisting on a word boundary at each end.

								.

								.

								.SS "Checking the PCRE2 version"

								.rs

								.sp

								Programs that link with a PCRE2 library can check the version by calling

								\fBpcre2_config()\fP with appropriate arguments. Users of applications that do

								not have access to the underlying code cannot do this. A special "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:

								.sp

								  (?(VERSION>=10.4)yes|no)

								.sp

								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.

								.

								.

								.SS "Assertion conditions"

								.rs

								.sp

								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 assertion, not one of the

								.\" HTML <a href="#nonatomicassertions">

								.\" </a>

								non-atomic assertions.

								.\"

								.P

								Consider this pattern, again containing non-significant white space, and with

								the two alternatives on the second line:

								.sp

								  (?(?=[^a-z]*[a-z])

								  \ed{2}-[a-z]{3}-\ed{2}  |  \ed{2}-\ed{2}-\ed{2} )

								.sp

								The condition is a positive lookahead assertion that matches an optional

								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 letter 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.

								.P

								When an assertion that is a condition contains capture groups, any

								capturing that occurs in a matching branch is retained afterwards, for both

								positive and negative assertions, because matching always continues after the

								assertion, whether it succeeds or fails. (Compare non-conditional assertions,

								for which captures are retained only for positive assertions that succeed.)

								.

								.

								.\" HTML <a name="comments"></a>

								.SH COMMENTS

								.rs

								.sp

								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.

								.P

								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

								.\" HTML <a href="#newlines">

								.\" </a>

								"Newline 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 single linefeed character) is in force:

								.sp

								  abc #comment \en still comment

								.sp

								On encountering the # character, \fBpcre2_compile()\fP skips along, looking for

								a newline in the pattern. The sequence \en 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.

								.

								.

								.\" HTML <a name="recursion"></a>

								.SH "RECURSIVE PATTERNS"

								.rs

								.sp

								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.

								.P

								For some time, Perl has provided a facility that allows regular expressions 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:

								.sp

								  $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;

								.sp

								The (?p{...}) item interpolates Perl code at run time, and in this case refers

								recursively to the pattern in which it appears.

								.P

								Obviously, PCRE2 cannot support the interpolation of Perl code. Instead, 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.

								.P

								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

								.\" HTML <a href="#groupsassubroutines">

								.\" </a>

								non-recursive subroutine

								.\"

								call, which is described in the next section.) The special item (?R) or (?0) is

								a recursive call of the entire regular expression.

								.P

								This PCRE2 pattern solves the nested parentheses problem (assume the

								PCRE2_EXTENDED option is set so that white space is ignored):

								.sp

								  \e( ( [^()]++ | (?R) )* \e)

								.sp

								First it matches an opening parenthesis. Then it matches any number of

								substrings which can either be a sequence of non-parentheses, or a recursive

								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.

								.P

								If this were part of a larger pattern, you would not want to recurse the entire

								pattern, so instead you could use this:

								.sp

								  ( \e( ( [^()]++ | (?1) )* \e) )

								.sp

								We have put the pattern into parentheses, and caused the recursion to refer to

								them instead of the whole pattern.

								.P

								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.

								.P

								Be aware however, that if

								.\" HTML <a href="#dupgroupnumber">

								.\" </a>

								duplicate capture group numbers

								.\"

								are in use, relative references refer to the earliest group with the

								appropriate number. Consider, for example:

								.sp

								  (?|(a)|(b)) (c) (?-2)

								.sp

								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, relative references are just a

								shorthand for computing a group number.

								.P

								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

								.\" HTML <a href="#groupsassubroutines">

								.\" </a>

								non-recursive subroutine

								.\"

								calls, as described in the next section.

								.P

								An alternative approach is to use named parentheses. The Perl syntax for this

								is (?&name); PCRE1's earlier syntax (?P>name) is also supported. We could

								rewrite the above example as follows:

								.sp

								  (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )

								.sp

								If there is more than one group with the same name, the earliest one is

								used.

								.P

								The example pattern that we have been looking at contains nested unlimited

								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

								.sp

								  (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

								.sp

								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.

								.P

								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

								.\" HREF

								\fBpcre2callout\fP

								.\"

								documentation). If the pattern above is matched against

								.sp

								  (ab(cd)ef)

								.sp

								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.

								.P

								Do not confuse the (?R) item with the condition (R), which tests for recursion.

								Consider this pattern, which matches text in angle brackets, allowing for

								arbitrary nesting. Only digits are allowed in nested brackets (that is, when

								recursing), whereas any characters are permitted at the outer level.

								.sp

								  < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >

								.sp

								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.

								.

								.

								.\" HTML <a name="recursiondifference"></a>

								.SS "Differences in recursion processing between PCRE2 and Perl"

								.rs

								.sp

								Some former differences between PCRE2 and Perl no longer exist.

								.P

								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.)

								.P

								Starting with release 10.30, recursive subroutine calls are no longer treated

								as atomic. That is, they can be re-entered to try unused alternatives 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.

								.P

								Supporting backtracking into recursions simplifies certain types of recursive

								pattern. For example, this pattern matches palindromic strings:

								.sp

								  ^((.)(?1)\e2|.?)$

								.sp

								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 pattern has to ignore all non-word characters,

								which can be done like this:

								.sp

								  ^\eW*+((.)\eW*+(?1)\eW*+\e2|\eW*+.?)\eW*+$

								.sp

								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 possessive 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.

								.P

								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:

								.sp

								  ^(.)(\e1|a(?2))

								.sp

								This pattern matches "bab". The first capturing parentheses match "b", then in

								the second group, when the backreference \e1 fails to match "b", the second

								alternative matches "a" and then recurses. In the recursion, \e1 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.

								.

								.

								.\" HTML <a name="groupsassubroutines"></a>

								.SH "GROUPS AS SUBROUTINES"

								.rs

								.sp

								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 defined before or after the reference. A

								numbered reference can be absolute or relative, as in these examples:

								.sp

								  (...(absolute)...)...(?2)...

								  (...(relative)...)...(?-1)...

								  (...(?+1)...(relative)...

								.sp

								An earlier example pointed out that the pattern

								.sp

								  (sens|respons)e and \e1ibility

								.sp

								matches "sense and sensibility" and "response and responsibility", but not

								"sense and responsibility". If instead the pattern

								.sp

								  (sens|respons)e and (?1)ibility

								.sp

								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.

								.P

								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.

								.P

								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:

								.sp

								  (abc)(?i:(?-1))

								.sp

								It matches "abcabc". It does not match "abcABC" because the change of

								processing option does not affect the called group.

								.P

								The behaviour of

								.\" HTML <a href="#backtrackcontrol">

								.\" </a>

								backtracking control verbs

								.\"

								in groups when called as subroutines is described in the section entitled

								.\" HTML <a href="#btsub">

								.\" </a>

								"Backtracking verbs in subroutines"

								.\"

								below.

								.

								.

								.\" HTML <a name="onigurumasubroutines"></a>

								.SH "ONIGURUMA SUBROUTINE SYNTAX"

								.rs

								.sp

								For compatibility with Oniguruma, the non-Perl syntax \eg 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 recursively. Here are two

								of the examples used above, rewritten using this syntax:

								.sp

								  (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )

								  (sens|respons)e and \eg'1'ibility

								.sp

								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:

								.sp

								  (abc)(?i:\eg<-1>)

								.sp

								Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP

								synonymous. The former is a backreference; the latter is a subroutine call.

								.

								.

								.SH CALLOUTS

								.rs

								.sp

								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 substrings that match the

								same pair of parentheses when there is a repetition.

								.P

								PCRE2 provides a similar feature, but of course it cannot obey arbitrary 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

								\fBpcre2_set_callout()\fP, and then passing that context to \fBpcre2_match()\fP

								or \fBpcre2_dfa_match()\fP. If no match context is passed, or if the callout

								entry point is set to NULL, callouts are disabled.

								.P

								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.

								.P

								During matching, when PCRE2 reaches a callout point, the external function 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.

								.P

								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

								.\" HREF

								\fBpcre2callout\fP

								.\"

								documentation.

								.

								.

								.SS "Callouts with numerical arguments"

								.rs

								.sp

								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:

								.sp

								  (?C1)abc(?C2)def

								.sp

								If the PCRE2_AUTO_CALLOUT flag is passed to \fBpcre2_compile()\fP, numerical

								callouts are automatically installed before each item in the pattern. They are

								all numbered 255. If there is a conditional group in the pattern 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:

								.sp

								  (?(?C9)(?=a)abc|def)

								.sp

								Note that this applies only to assertion conditions, not to other types of

								condition.

								.

								.

								.SS "Callouts with string arguments"

								.rs

								.sp

								A delimited string may be used instead of a number as a callout argument. The

								starting delimiter must be one of ` ' " ^ % # $ { and the ending delimiter is

								the same as the start, except for {, where the ending delimiter is }. If the

								ending delimiter is needed within the string, it must be doubled. For

								example:

								.sp

								  (?C'ab ''c'' d')xyz(?C{any text})pqr

								.sp

								The doubling is removed before the string is passed to the callout function.

								.

								.

								.\" HTML <a name="backtrackcontrol"></a>

								.SH "BACKTRACKING CONTROL"

								.rs

								.sp

								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.

								.P

								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-compatible.

								.P

								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 \eQ, \eE, and sequences such as

								\ex{100} that define character code points. Character type escapes such as \ed

								are faulted.

								.P

								A closing parenthesis can be included in a name either as \e) or between \eQ

								and \eE. 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.

								.P

								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 pattern. Except for

								(*ACCEPT), they may not be quantified.

								.P

								Since these verbs are specifically related to backtracking, most of them can be

								used only when the pattern is to be matched using the traditional 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.

								.P

								The behaviour of these verbs in

								.\" HTML <a href="#btrepeat">

								.\" </a>

								repeated groups,

								.\"

								.\" HTML <a href="#btassert">

								.\" </a>

								assertions,

								.\"

								and in

								.\" HTML <a href="#btsub">

								.\" </a>

								capture groups called as subroutines

								.\"

								(whether or not recursively) is documented below.

								.

								.

								.\" HTML <a name="nooptimize"></a>

								.SS "Optimizations that affect backtracking verbs"

								.rs

								.sp

								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 \fBpcre2_compile()\fP, or by starting the pattern with

								(*NO_START_OPT). There is more discussion of this option in the section

								entitled

								.\" HTML <a href="pcre2api.html#compiling">

								.\" </a>

								"Compiling a pattern"

								.\"

								in the

								.\" HREF

								\fBpcre2api\fP

								.\"

								documentation.

								.P

								Experiments with Perl suggest that it too has similar optimizations, and like

								PCRE2, turning them off can change the result of a match.

								.

								.

								.\" HTML <a name="acceptverb"></a>

								.SS "Verbs that act immediately"

								.rs

								.sp

								The following verbs act as soon as they are encountered.

								.sp

								   (*ACCEPT) or (*ACCEPT:NAME)

								.sp

								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 positive assertion, the

								assertion succeeds; in a negative assertion, the assertion fails.

								.P

								If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For

								example:

								.sp

								  A((?:A|B(*ACCEPT)|C)D)

								.sp

								This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by

								the outer parentheses.

								.P

								(*ACCEPT) is the only backtracking verb that is allowed to be quantified

								because an ungreedy quantification with a minimum of zero acts only when a

								backtrack happens. Consider, for example,

								.sp

								  (A(*ACCEPT)??B)C

								.sp

								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 captured. Whereas (*COMMIT)

								(see below) means "fail on backtrack", a repeated (*ACCEPT) of this type means

								"succeed on backtrack".

								.P

								\fBWarning:\fP (*ACCEPT) should not be used within a script run group, because

								it causes an immediate exit from the group, bypassing the script run checking.

								.sp

								  (*FAIL) or (*FAIL:NAME)

								.sp

								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 feature, as for example in this pattern:

								.sp

								  a+(?C)(*FAIL)

								.sp

								A match with the string "aaaa" always fails, but the callout is taken before

								each backtrack happens (in this example, 10 times).

								.P

								(*ACCEPT:NAME) and (*FAIL:NAME) behave the same as (*MARK:NAME)(*ACCEPT) and

								(*MARK:NAME)(*FAIL), respectively, that is, a (*MARK) is recorded just before

								the verb acts.

								.

								.

								.SS "Recording which path was taken"

								.rs

								.sp

								There is one verb whose main purpose is to track how a match was arrived at,

								though it also has a secondary use in conjunction with advancing the match

								starting point (see (*SKIP) below).

								.sp

								  (*MARK:NAME) or (*:NAME)

								.sp

								A name is always required with this verb. For all the other backtracking

								control verbs, a NAME argument is optional.

								.P

								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 section entitled

								.\" HTML <a href="pcre2api.html#matchotherdata">

								.\" </a>

								"Other information about the match"

								.\"

								in the

								.\" HREF

								\fBpcre2api\fP

								.\"

								documentation. 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.

								.P

								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

								\fBpcre2test\fP output, where the "mark" modifier requests the retrieval and

								outputting of (*MARK) data:

								.sp

								    re> /X(*MARK:A)Y|X(*MARK:B)Z/mark

								  data> XY

								   0: XY

								  MK: A

								  XZ

								   0: XZ

								  MK: B

								.sp

								The (*MARK) name is tagged with "MK:" in this output, and in this example it

								indicates which of the two alternatives matched. This is a more efficient way

								of obtaining this information than putting each alternative in its own

								capturing parentheses.

								.P

								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-encountered. This does not

								happen for negative assertions or failing positive assertions.

								.P

								After a partial match or a failed match, the last encountered name in the

								entire match process is returned. For example:

								.sp

								    re> /X(*MARK:A)Y|X(*MARK:B)Z/mark

								  data> XP

								  No match, mark = B

								.sp

								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.

								.P

								If you are interested in (*MARK) values after failed matches, you should

								probably set the PCRE2_NO_START_OPTIMIZE option

								.\" HTML <a href="#nooptimize">

								.\" </a>

								(see above)

								.\"

								to ensure that the match is always attempted.

								.

								.

								.SS "Verbs that act after backtracking"

								.rs

								.sp

								The following verbs do nothing when they are encountered. Matching continues

								with what follows, but if there is a subsequent match failure, causing a

								backtrack to the verb, a failure is forced. That is, backtracking 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. Backtracking from beyond an assertion or an atomic group

								ignores the entire group, and seeks a preceding backtracking point.

								.P

								These verbs differ in exactly what kind of failure occurs when backtracking

								reaches them. The behaviour described below is what happens when the verb is

								not in a subroutine or an assertion. Subsequent sections cover these special

								cases.

								.sp

								  (*COMMIT) or (*COMMIT:NAME)

								.sp

								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 pattern 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 \fBpcre2_match()\fP is committed to finding a match at

								the current starting point, or not at all. For example:

								.sp

								  a+(*COMMIT)b

								.sp

								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."

								.P

								The behaviour of (*COMMIT:NAME) is not the same as (*MARK:NAME)(*COMMIT). It is

								like (*MARK:NAME) in that the name is remembered for passing back to the

								caller. However, (*SKIP:NAME) searches only for names that are set with

								(*MARK), ignoring those set by any of the other backtracking verbs.

								.P

								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.

								.P

								Note that (*COMMIT) at the start of a pattern is not the same as an anchor,

								unless PCRE2's start-of-match optimizations are turned off, as shown in this

								output from \fBpcre2test\fP:

								.sp

								    re> /(*COMMIT)abc/

								  data> xyzabc

								   0: abc

								  data>

								  re> /(*COMMIT)abc/no_start_optimize

								  data> xyzabc

								  No match

								.sp

								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.

								.sp

								  (*PRUNE) or (*PRUNE:NAME)

								.sp

								This verb causes the match to fail at the current starting position in the

								subject if there is a later matching failure that causes backtracking 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 quantifier, 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).

								.P

								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.

								.sp

								  (*SKIP)

								.sp

								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 encountered. (*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:

								.sp

								  a+(*SKIP)b

								.sp

								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 quantifier does not have the same

								effect as this example; although it would suppress backtracking during the

								first match attempt, the second attempt would start at the second character

								instead of skipping on to "c".

								.P

								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.

								.sp

								  (*SKIP:NAME)

								.sp

								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 corresponds to that (*MARK) instead of

								to where (*SKIP) was encountered. If no (*MARK) with a matching name is found,

								the (*SKIP) is ignored.

								.P

								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 backtracking. Compare the

								following \fBpcre2test\fP examples:

								.sp

								    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

								.sp

								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 second 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.

								.P

								Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores

								names that are set by other backtracking verbs.

								.sp

								  (*THEN) or (*THEN:NAME)

								.sp

								This verb causes a skip to the next innermost alternative when backtracking

								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:

								.sp

								  ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

								.sp

								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 subsequently BAZ fails, there are no

								more alternatives, so there is a backtrack to whatever came before the entire

								group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).

								.P

								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.

								.P

								A group that does not contain a | character is just a part of the enclosing

								alternative; it is not a nested alternation with only one alternative. The

								effect of (*THEN) extends beyond such a group to the enclosing alternative.

								Consider this pattern, where A, B, etc. are complex pattern fragments that do

								not contain any | characters at this level:

								.sp

								  A (B(*THEN)C) | D

								.sp

								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:

								.sp

								  A (B(*THEN)C | (*FAIL)) | D

								.sp

								The effect of (*THEN) is now confined to the inner group. After a failure 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.

								.P

								Note that a conditional group is not considered as having two alternatives,

								because only one is ever used. In other words, the | character in a conditional

								group has a different meaning. Ignoring white space, consider:

								.sp

								  ^.*? (?(?=a) a | b(*THEN)c )

								.sp

								If the subject is "ba", this pattern does not match. Because .*? is ungreedy,

								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.)

								.P

								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.

								.

								.

								.SS "More than one backtracking verb"

								.rs

								.sp

								If more than one backtracking verb is present in a pattern, the one that is

								backtracked onto first acts. For example, consider this pattern, where A, B,

								etc. are complex pattern fragments:

								.sp

								  (A(*COMMIT)B(*THEN)C|ABD)

								.sp

								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:

								.sp

								  ...(*COMMIT)(*PRUNE)...

								.sp

								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).

								.

								.

								.\" HTML <a name="btrepeat"></a>

								.SS "Backtracking verbs in repeated groups"

								.rs

								.sp

								PCRE2 sometimes differs from Perl in its handling of backtracking verbs in

								repeated groups. For example, consider:

								.sp

								  /(a(*COMMIT)b)+ac/

								.sp

								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.

								.

								.

								.\" HTML <a name="btassert"></a>

								.SS "Backtracking verbs in assertions"

								.rs

								.sp

								(*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.

								.P

								(*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, (*ACCEPT) causes the assertion to

								fail without any further processing; captured substrings and any mark name are

								discarded.

								.P

								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.

								.P

								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.

								.P

								PCRE2 now supports non-atomic positive assertions, as described in the section

								entitled

								.\" HTML <a href="#nonatomicassertions">

								.\" </a>

								"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 assertion, and therefore verbs such as (*COMMIT) can be triggered by

								backtracks from later in the pattern.

								.P

								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.

								.P

								The other backtracking verbs are not treated specially if they appear in a

								standalone positive assertion. In a conditional positive assertion,

								backtracking (from within the assertion) into (*COMMIT), (*SKIP), or (*PRUNE)

								causes the condition to be false. However, for both standalone and conditional

								negative assertions, backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes

								the assertion to be true, without considering any further alternative branches.

								.

								.

								.\" HTML <a name="btsub"></a>

								.SS "Backtracking verbs in subroutines"

								.rs

								.sp

								These behaviours occur whether or not the group is called recursively.

								.P

								(*ACCEPT) in a group called as a subroutine causes the subroutine match to

								succeed without any further processing. Matching then continues after the

								subroutine call. Perl documents this behaviour. Perl's treatment of the other

								verbs in subroutines is different in some cases.

								.P

								(*FAIL) in a group called as a subroutine has its normal effect: it forces

								an immediate backtrack.

								.P

								(*COMMIT), (*SKIP), and (*PRUNE) cause the subroutine match to fail when

								triggered by being backtracked to in a group called as a subroutine. There is

								then a backtrack at the outer level.

								.P

								(*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.

								.

								.

								.SH "SEE ALSO"

								.rs

								.sp

								\fBpcre2api\fP(3), \fBpcre2callout\fP(3), \fBpcre2matching\fP(3),

								\fBpcre2syntax\fP(3), \fBpcre2\fP(3).

								.

								.

								.SH AUTHOR

								.rs

								.sp

								.nf

								Philip Hazel

								Retired from University Computing Service

								Cambridge, England.

								.fi

								.

								.

								.SH REVISION

								.rs

								.sp

								.nf

								Last updated: 04 June 2024

								Copyright (c) 1997-2024 University of Cambridge.

								.fi