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<html><head><title>pcre2callout specification</title></head><body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"><h1>pcre2callout man page</h1><p>Return to the <a href="index.html">PCRE2 index page</a>.</p><p>This page is part of the PCRE2 HTML documentation. It was generatedautomatically from the original man page. If there is any nonsense in it,please consult the man page, in case the conversion went wrong.<br><ul><li><a name="TOC1" href="#SEC1">SYNOPSIS</a><li><a name="TOC2" href="#SEC2">DESCRIPTION</a><li><a name="TOC3" href="#SEC3">MISSING CALLOUTS</a><li><a name="TOC4" href="#SEC4">THE CALLOUT INTERFACE</a><li><a name="TOC5" href="#SEC5">RETURN VALUES FROM CALLOUTS</a><li><a name="TOC6" href="#SEC6">CALLOUT ENUMERATION</a><li><a name="TOC7" href="#SEC7">AUTHOR</a><li><a name="TOC8" href="#SEC8">REVISION</a></ul><br><a name="SEC1" href="#TOC1">SYNOPSIS</a><br><P><b>#include <pcre2.h></b></P><P><b>int (*pcre2_callout)(pcre2_callout_block *, void *);</b><br><br><b>int pcre2_callout_enumerate(const pcre2_code *<i>code</i>,</b><b> int (*<i>callback</i>)(pcre2_callout_enumerate_block *, void *),</b><b> void *<i>user_data</i>);</b></P><br><a name="SEC2" href="#TOC1">DESCRIPTION</a><br><P>PCRE2 provides a feature called "callout", which is a means of temporarilypassing control to the caller of PCRE2 in the middle of pattern matching. Thecaller of PCRE2 provides an external function by putting its entry point ina match context (see <b>pcre2_set_callout()</b> in the<a href="pcre2api.html"><b>pcre2api</b></a>documentation).</P><P>When using the <b>pcre2_substitute()</b> function, an additional callout featureis available. This does a callout after each change to the subject string andis described in the<a href="pcre2api.html"><b>pcre2api</b></a>documentation; the rest of this document is concerned with callouts duringpattern matching.</P><P>Within a regular expression, (?C<arg>) indicates a point at which the externalfunction is to be called. Different callout points can be identified by puttinga number less than 256 after the letter C. The default value is zero.Alternatively, the argument may be a delimited string. The starting delimitermust be one of ` ' " ^ % # $ { and the ending delimiter is the same as thestart, except for {, where the ending delimiter is }. If the ending delimiteris needed within the string, it must be doubled. For example, this pattern hastwo callout points:<pre> (?C1)abc(?C"some ""arbitrary"" text")def</pre>If the PCRE2_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE2automatically inserts callouts, all with number 255, before each item in thepattern except for immediately before or after an explicit callout. Forexample, if PCRE2_AUTO_CALLOUT is used with the pattern<pre> A(?C3)B</pre>it is processed as if it were<pre> (?C255)A(?C3)B(?C255)</pre>Here is a more complicated example:<pre> A(\d{2}|--)</pre>With PCRE2_AUTO_CALLOUT, this pattern is processed as if it were<pre> (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)</pre>Notice that there is a callout before and after each parenthesis andalternation bar. If the pattern contains a conditional group whose condition isan assertion, an automatic callout is inserted immediately before thecondition. Such a callout may also be inserted explicitly, for example:<pre> (?(?C9)(?=a)ab|de) (?(?C%text%)(?!=d)ab|de)</pre>This applies only to assertion conditions (because they are themselvesindependent groups).</P><P>Callouts can be useful for tracking the progress of pattern matching. The<a href="pcre2test.html"><b>pcre2test</b></a>program has a pattern qualifier (/auto_callout) that sets automatic callouts.When any callouts are present, the output from <b>pcre2test</b> indicates howthe pattern is being matched. This is useful information when you are trying tooptimize the performance of a particular pattern.</P><br><a name="SEC3" href="#TOC1">MISSING CALLOUTS</a><br><P>You should be aware that, because of optimizations in the way PCRE2 compilesand matches patterns, callouts sometimes do not happen exactly as you mightexpect.</P><br><b>Auto-possessification</b><br><P>At compile time, PCRE2 "auto-possessifies" repeated items when it knows thatwhat follows cannot be part of the repeat. For example, a+[bc] is compiled asif it were a++[bc]. The <b>pcre2test</b> output when this pattern is compiledwith PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied to the string"aaaa" is:<pre> --->aaaa +0 ^ a+ +2 ^ ^ [bc] No match</pre>This indicates that when matching [bc] fails, there is no backtracking into a+(because it is being treated as a++) and therefore the callouts that would betaken for the backtracks do not occur. You can disable the auto-possessifyfeature by passing PCRE2_NO_AUTO_POSSESS to <b>pcre2_compile()</b>, or startingthe pattern with (*NO_AUTO_POSSESS). In this case, the output changes to this:<pre> --->aaaa +0 ^ a+ +2 ^ ^ [bc] +2 ^ ^ [bc] +2 ^ ^ [bc] +2 ^^ [bc] No match</pre>This time, when matching [bc] fails, the matcher backtracks into a+ and triesagain, repeatedly, until a+ itself fails.</P><br><b>Automatic .* anchoring</b><br><P>By default, an optimization is applied when .* is the first significant item ina pattern. If PCRE2_DOTALL is set, so that the dot can match any character, thepattern is automatically anchored. If PCRE2_DOTALL is not set, a match canstart only after an internal newline or at the beginning of the subject, and<b>pcre2_compile()</b> remembers this. If a pattern has more than one top-levelbranch, automatic anchoring occurs if all branches are anchorable.</P><P>This optimization is disabled, however, if .* is in an atomic group or if thereis a backreference to the capture group in which it appears. It is alsodisabled if the pattern contains (*PRUNE) or (*SKIP). However, the presence ofcallouts does not affect it.</P><P>For example, if the pattern .*\d is compiled with PCRE2_AUTO_CALLOUT andapplied to the string "aa", the <b>pcre2test</b> output is:<pre> --->aa +0 ^ .* +2 ^ ^ \d +2 ^^ \d +2 ^ \d No match</pre>This shows that all match attempts start at the beginning of the subject. Inother words, the pattern is anchored. You can disable this optimization bypassing PCRE2_NO_DOTSTAR_ANCHOR to <b>pcre2_compile()</b>, or starting thepattern with (*NO_DOTSTAR_ANCHOR). In this case, the output changes to:<pre> --->aa +0 ^ .* +2 ^ ^ \d +2 ^^ \d +2 ^ \d +0 ^ .* +2 ^^ \d +2 ^ \d No match</pre>This shows more match attempts, starting at the second subject character.Another optimization, described in the next section, means that there is nosubsequent attempt to match with an empty subject.</P><br><b>Other optimizations</b><br><P>Other optimizations that provide fast "no match" results also affect callouts.For example, if the pattern is<pre> ab(?C4)cd</pre>PCRE2 knows that any matching string must contain the letter "d". If thesubject string is "abyz", the lack of "d" means that matching doesn't everstart, and the callout is never reached. However, with "abyd", though theresult is still no match, the callout is obeyed.</P><P>For most patterns PCRE2 also knows the minimum length of a matching string, andwill immediately give a "no match" return without actually running a match ifthe subject is not long enough, or, for unanchored patterns, if it has beenscanned far enough.</P><P>You can disable these optimizations by passing the PCRE2_NO_START_OPTIMIZEoption to <b>pcre2_compile()</b>, or by starting the pattern with(*NO_START_OPT). This slows down the matching process, but does ensure thatcallouts such as the example above are obeyed.<a name="calloutinterface"></a></P><br><a name="SEC4" href="#TOC1">THE CALLOUT INTERFACE</a><br><P>During matching, when PCRE2 reaches a callout point, if an external function isprovided in the match context, it is called. This applies to both normal,DFA, and JIT matching. The first argument to the callout function is a pointerto a <b>pcre2_callout</b> block. The second argument is the void * callout datathat was supplied when the callout was set up by calling<b>pcre2_set_callout()</b> (see the<a href="pcre2api.html"><b>pcre2api</b></a>documentation). The callout block structure contains the following fields, notnecessarily in this order:<pre> uint32_t <i>version</i>; uint32_t <i>callout_number</i>; uint32_t <i>capture_top</i>; uint32_t <i>capture_last</i>; uint32_t <i>callout_flags</i>; PCRE2_SIZE *<i>offset_vector</i>; PCRE2_SPTR <i>mark</i>; PCRE2_SPTR <i>subject</i>; PCRE2_SIZE <i>subject_length</i>; PCRE2_SIZE <i>start_match</i>; PCRE2_SIZE <i>current_position</i>; PCRE2_SIZE <i>pattern_position</i>; PCRE2_SIZE <i>next_item_length</i>; PCRE2_SIZE <i>callout_string_offset</i>; PCRE2_SIZE <i>callout_string_length</i>; PCRE2_SPTR <i>callout_string</i>;</pre>The <i>version</i> field contains the version number of the block format. Thecurrent version is 2; the three callout string fields were added for version 1,and the <i>callout_flags</i> field for version 2. If you are writing anapplication that might use an earlier release of PCRE2, you should check theversion number before accessing any of these fields. The version number willincrease in future if more fields are added, but the intention is never toremove any of the existing fields.</P><br><b>Fields for numerical callouts</b><br><P>For a numerical callout, <i>callout_string</i> is NULL, and <i>callout_number</i>contains the number of the callout, in the range 0-255. This is the numberthat follows (?C for callouts that part of the pattern; it is 255 forautomatically generated callouts.</P><br><b>Fields for string callouts</b><br><P>For callouts with string arguments, <i>callout_number</i> is always zero, and<i>callout_string</i> points to the string that is contained within the compiledpattern. Its length is given by <i>callout_string_length</i>. Duplicated endingdelimiters that were present in the original pattern string have been turnedinto single characters, but there is no other processing of the callout stringargument. An additional code unit containing binary zero is present after thestring, but is not included in the length. The delimiter that was used to startthe string is also stored within the pattern, immediately before the stringitself. You can access this delimiter as <i>callout_string</i>[-1] if you needit.</P><P>The <i>callout_string_offset</i> field is the code unit offset to the start ofthe callout argument string within the original pattern string. This isprovided for the benefit of applications such as script languages that mightneed to report errors in the callout string within the pattern.</P><br><b>Fields for all callouts</b><br><P>The remaining fields in the callout block are the same for both kinds ofcallout.</P><P>The <i>offset_vector</i> field is a pointer to a vector of capturing offsets(the "ovector"). You may read the elements in this vector, but you must notchange any of them.</P><P>For calls to <b>pcre2_match()</b>, the <i>offset_vector</i> field is not (sincerelease 10.30) a pointer to the actual ovector that was passed to the matchingfunction in the match data block. Instead it points to an internal ovector of asize large enough to hold all possible captured substrings in the pattern. Notethat whenever a recursion or subroutine call within a pattern completes, thecapturing state is reset to what it was before.</P><P>The <i>capture_last</i> field contains the number of the most recently capturedsubstring, and the <i>capture_top</i> field contains one more than the number ofthe highest numbered captured substring so far. If no substrings have yet beencaptured, the value of <i>capture_last</i> is 0 and the value of<i>capture_top</i> is 1. The values of these fields do not always differ by one;for example, when the callout in the pattern ((a)(b))(?C2) is taken,<i>capture_last</i> is 1 but <i>capture_top</i> is 4.</P><P>The contents of ovector[2] to ovector[<capture_top>*2-1] can be inspected inorder to extract substrings that have been matched so far, in the same way asextracting substrings after a match has completed. The values in ovector[0] andovector[1] are always PCRE2_UNSET because the match is by definition notcomplete. Substrings that have not been captured but whose numbers are lessthan <i>capture_top</i> also have both of their ovector slots set toPCRE2_UNSET.</P><P>For DFA matching, the <i>offset_vector</i> field points to the ovector that waspassed to the matching function in the match data block for callouts at the toplevel, but to an internal ovector during the processing of pattern recursions,lookarounds, and atomic groups. However, these ovectors hold no usefulinformation because <b>pcre2_dfa_match()</b> does not support substringcapturing. The value of <i>capture_top</i> is always 1 and the value of<i>capture_last</i> is always 0 for DFA matching.</P><P>The <i>subject</i> and <i>subject_length</i> fields contain copies of the valuesthat were passed to the matching function.</P><P>The <i>start_match</i> field normally contains the offset within the subject atwhich the current match attempt started. However, if the escape sequence \Khas been encountered, this value is changed to reflect the modified startingpoint. If the pattern is not anchored, the callout function may be calledseveral times from the same point in the pattern for different starting pointsin the subject.</P><P>The <i>current_position</i> field contains the offset within the subject of thecurrent match pointer.</P><P>The <i>pattern_position</i> field contains the offset in the pattern string tothe next item to be matched.</P><P>The <i>next_item_length</i> field contains the length of the next item to beprocessed in the pattern string. When the callout is at the end of the pattern,the length is zero. When the callout precedes an opening parenthesis, thelength includes meta characters that follow the parenthesis. For example, in acallout before an assertion such as (?=ab) the length is 3. For an alternationbar or a closing parenthesis, the length is one, unless a closing parenthesisis followed by a quantifier, in which case its length is included. (Thischanged in release 10.23. In earlier releases, before an opening parenthesisthe length was that of the entire group, and before an alternation bar or aclosing parenthesis the length was zero.)</P><P>The <i>pattern_position</i> and <i>next_item_length</i> fields are intended tohelp in distinguishing between different automatic callouts, which all have thesame callout number. However, they are set for all callouts, and are used by<b>pcre2test</b> to show the next item to be matched when displaying calloutinformation.</P><P>In callouts from <b>pcre2_match()</b> the <i>mark</i> field contains a pointer tothe zero-terminated name of the most recently passed (*MARK), (*PRUNE), or(*THEN) item in the match, or NULL if no such items have been passed. Instancesof (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). Incallouts from the DFA matching function this field always contains NULL.</P><P>The <i>callout_flags</i> field is always zero in callouts from<b>pcre2_dfa_match()</b> or when JIT is being used. When <b>pcre2_match()</b>without JIT is used, the following bits may be set:<pre> PCRE2_CALLOUT_STARTMATCH</pre>This is set for the first callout after the start of matching for each newstarting position in the subject.<pre> PCRE2_CALLOUT_BACKTRACK</pre>This is set if there has been a matching backtrack since the previous callout,or since the start of matching if this is the first callout from a<b>pcre2_match()</b> run.</P><P>Both bits are set when a backtrack has caused a "bumpalong" to a new startingposition in the subject. Output from <b>pcre2test</b> does not indicate thepresence of these bits unless the <b>callout_extra</b> modifier is set.</P><P>The information in the <b>callout_flags</b> field is provided so thatapplications can track and tell their users how matching with backtracking isdone. This can be useful when trying to optimize patterns, or just tounderstand how PCRE2 works. There is no support in <b>pcre2_dfa_match()</b>because there is no backtracking in DFA matching, and there is no support inJIT because JIT is all about maximimizing matching performance. In both thesecases the <b>callout_flags</b> field is always zero.</P><br><a name="SEC5" href="#TOC1">RETURN VALUES FROM CALLOUTS</a><br><P>The external callout function returns an integer to PCRE2. If the value iszero, matching proceeds as normal. If the value is greater than zero, matchingfails at the current point, but the testing of other matching possibilitiesgoes ahead, just as if a lookahead assertion had failed. If the value is lessthan zero, the match is abandoned, and the matching function returns thenegative value.</P><P>Negative values should normally be chosen from the set of PCRE2_ERROR_xxxvalues. In particular, PCRE2_ERROR_NOMATCH forces a standard "no match"failure. The error number PCRE2_ERROR_CALLOUT is reserved for use by calloutfunctions; it will never be used by PCRE2 itself.</P><br><a name="SEC6" href="#TOC1">CALLOUT ENUMERATION</a><br><P><b>int pcre2_callout_enumerate(const pcre2_code *<i>code</i>,</b><b> int (*<i>callback</i>)(pcre2_callout_enumerate_block *, void *),</b><b> void *<i>user_data</i>);</b><br><br>A script language that supports the use of string arguments in callouts mightlike to scan all the callouts in a pattern before running the match. This canbe done by calling <b>pcre2_callout_enumerate()</b>. The first argument is apointer to a compiled pattern, the second points to a callback function, andthe third is arbitrary user data. The callback function is called for everycallout in the pattern in the order in which they appear. Its first argument isa pointer to a callout enumeration block, and its second argument is the<i>user_data</i> value that was passed to <b>pcre2_callout_enumerate()</b>. Thedata block contains the following fields:<pre> <i>version</i> Block version number <i>pattern_position</i> Offset to next item in pattern <i>next_item_length</i> Length of next item in pattern <i>callout_number</i> Number for numbered callouts <i>callout_string_offset</i> Offset to string within pattern <i>callout_string_length</i> Length of callout string <i>callout_string</i> Points to callout string or is NULL</pre>The version number is currently 0. It will increase if new fields are everadded to the block. The remaining fields are the same as their namesakes in the<b>pcre2_callout</b> block that is used for callouts during matching, asdescribed<a href="#calloutinterface">above.</a></P><P>Note that the value of <i>pattern_position</i> is unique for each callout.However, if a callout occurs inside a group that is quantified with a non-zerominimum or a fixed maximum, the group is replicated inside the compiledpattern. For example, a pattern such as /(a){2}/ is compiled as if it were/(a)(a)/. This means that the callout will be enumerated more than once, butwith the same value for <i>pattern_position</i> in each case.</P><P>The callback function should normally return zero. If it returns a non-zerovalue, scanning the pattern stops, and that value is returned from<b>pcre2_callout_enumerate()</b>.</P><br><a name="SEC7" href="#TOC1">AUTHOR</a><br><P>Philip Hazel<br>Retired from University Computing Service<br>Cambridge, England.<br></P><br><a name="SEC8" href="#TOC1">REVISION</a><br><P>Last updated: 19 January 2024<br>Copyright © 1997-2024 University of Cambridge.<br><p>Return to the <a href="index.html">PCRE2 index page</a>.</p>
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