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<!-- manual page source format generated by PolyglotMan v3.2, -->
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<title>PCRE(3) manual page</title>
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<a href='#toc'>Table of Contents</a><p>
<h2><a name='sect0' href='#toc0'>Name</a></h2>
PCRE - Perl-compatible regular expressions
<h2><a name='sect1' href='#toc1'>Pcre Callouts</a></h2>
<p>
<b>int (*pcre_callout)(pcre_callout_block
*);</b> <p>
PCRE provides a feature called "callout", which is a means of temporarily
passing control to the caller of PCRE in the middle of pattern matching.
The caller of PCRE provides an external function by putting its entry point
in the global variable <i>pcre_callout</i>. By default, this variable contains
NULL, which disables all calling out. <p>
Within a regular expression, (?C)
indicates the points at which the external function is to be called. Different
callout points can be identified by putting a number less than 256 after
the letter C. The default value is zero. For example, this pattern has two
callout points: <p>
(?C1)deabc(?C2)def<br>
<p>
If the PCRE_AUTO_CALLOUT option bit is set when <b>pcre_compile()</b> is called,
PCRE automatically inserts callouts, all with number 255, before each item
in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
<p>
A(\d{2}|--)<br>
<p>
it is processed as if it were <p>
(?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
<p>
Notice that there is a callout before and after each parenthesis and alternation
bar. Automatic callouts can be used for tracking the progress of pattern
matching. The <b>pcretest</b> command has an option that sets automatic callouts;
when it is used, the output indicates how the pattern is matched. This is
useful information when you are trying to optimize the performance of a
particular pattern.
<h2><a name='sect2' href='#toc2'>Missing Callouts</a></h2>
<p>
You should be aware that, because
of optimizations in the way PCRE matches patterns, callouts sometimes do
not happen. For example, if the pattern is <p>
ab(?C4)cd<br>
<p>
PCRE knows that any matching string must contain the letter "d". If the
subject string is "abyz", the lack of "d" means that matching doesn’t ever
start, and the callout is never reached. However, with "abyd", though the
result is still no match, the callout is obeyed.
<h2><a name='sect3' href='#toc3'>the Callout Interface</a></h2>
<p>
During matching, when PCRE reaches a callout point, the external function
defined by <i>pcre_callout</i> is called (if it is set). The only argument is a
pointer to a <b>pcre_callout</b> block. This structure contains the following fields:
<p>
int <i>version</i>;<br>
int <i>callout_number</i>;<br>
int *<i>offset_vector</i>;<br>
const char *<i>subject</i>;<br>
int <i>subject_length</i>;<br>
int <i>start_match</i>;<br>
int <i>current_position</i>;<br>
int <i>capture_top</i>;<br>
int <i>capture_last</i>;<br>
void *<i>callout_data</i>;<br>
int <i>pattern_position</i>;<br>
int <i>next_item_length</i>;<br>
<p>
The <i>version</i> field is an integer containing the version number of the block
format. The initial version was 0; the current version is 1. The version
number will change again in future if additional fields are added, but
the intention is never to remove any of the existing fields. <p>
The <i>callout_number</i>
field contains the number of the callout, as compiled into the pattern
(that is, the number after ?C for manual callouts, and 255 for automatically
generated callouts). <p>
The <i>offset_vector</i> field is a pointer to the vector
of offsets that was passed by the caller to <b>pcre_exec()</b>. The contents can
be inspected in order to extract substrings that have been matched so far,
in the same way as for extracting substrings after a match has completed.
<p>
The <i>subject</i> and <i>subject_length</i> fields contain copies of the values that
were passed to <b>pcre_exec()</b>. <p>
The <i>start_match</i> field contains the offset within
the subject at which the current match attempt started. If the pattern is
not anchored, the callout function may be called several times from the
same point in the pattern for different starting points in the subject.
<p>
The <i>current_position</i> field contains the offset within the subject of the
current match pointer. <p>
The <i>capture_top</i> field contains one more than the
number of the highest numbered captured substring so far. If no substrings
have been captured, the value of <i>capture_top</i> is one. <p>
The <i>capture_last</i> field
contains the number of the most recently captured substring. If no substrings
have been captured, its value is -1. <p>
The <i>callout_data</i> field contains a value
that is passed to <b>pcre_exec()</b> by the caller specifically so that it can
be passed back in callouts. It is passed in the <i>pcre_callout</i> field of the
<b>pcre_extra</b> data structure. If no such data was passed, the value of <i>callout_data</i>
in a <b>pcre_callout</b> block is NULL. There is a description of the <b>pcre_extra</b>
structure in the <b>pcreapi</b> documentation. <p>
The <i>pattern_position</i> field is
present from version 1 of the <i>pcre_callout</i> structure. It contains the offset
to the next item to be matched in the pattern string. <p>
The <i>next_item_length</i>
field is present from version 1 of the <i>pcre_callout</i> structure. It contains
the length of the next item to be matched in the pattern string. When the
callout immediately precedes an alternation bar, a closing parenthesis,
or the end of the pattern, the length is zero. When the callout precedes
an opening parenthesis, the length is that of the entire subpattern. <p>
The
<i>pattern_position</i> and <i>next_item_length</i> fields are intended to help in distinguishing
between different automatic callouts, which all have the same callout number.
However, they are set for all callouts.
<h2><a name='sect4' href='#toc4'>Return Values</a></h2>
<p>
The external callout
function returns an integer to PCRE. If the value is zero, matching proceeds
as normal. If the value is greater than zero, matching fails at the current
point, but backtracking to test other matching possibilities goes ahead,
just as if a lookahead assertion had failed. If the value is less than zero,
the match is abandoned, and <b>pcre_exec()</b> returns the negative value. <p>
Negative
values should normally be chosen from the set of PCRE_ERROR_xxx values.
In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
it will never be used by PCRE itself. <p>
Last updated: 09 September 2004 <br>
Copyright (c) 1997-2004 University of Cambridge. <p>
<hr><p>
<a name='toc'><b>Table of Contents</b></a><p>
<ul>
<li><a name='toc0' href='#sect0'>Name</a></li>
<li><a name='toc1' href='#sect1'>Pcre Callouts</a></li>
<li><a name='toc2' href='#sect2'>Missing Callouts</a></li>
<li><a name='toc3' href='#sect3'>the Callout Interface</a></li>
<li><a name='toc4' href='#sect4'>Return Values</a></li>
</ul>
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