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+<html>
+<head>
+<title>pcrecallout specification</title>
+</head>
+<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
+<h1>pcrecallout man page</h1>
+<p>
+Return to the <a href="index.html">PCRE index page</a>.
+</p>
+<p>
+This page is part of the PCRE HTML documentation. It was generated automatically
+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">PCRE CALLOUTS</a>
+<li><a name="TOC2" href="#SEC2">MISSING CALLOUTS</a>
+<li><a name="TOC3" href="#SEC3">THE CALLOUT INTERFACE</a>
+<li><a name="TOC4" href="#SEC4">RETURN VALUES</a>
+<li><a name="TOC5" href="#SEC5">AUTHOR</a>
+<li><a name="TOC6" href="#SEC6">REVISION</a>
+</ul>
+<br><a name="SEC1" href="#TOC1">PCRE CALLOUTS</a><br>
+<P>
+<b>int (*pcre_callout)(pcre_callout_block *);</b>
+</P>
+<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>
+<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:
+<pre>
+ (?C1)abc(?C2)def
+</pre>
+If the PCRE_AUTO_CALLOUT option bit is set when <b>pcre_compile()</b> or
+<b>pcre_compile2()</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
+<pre>
+ A(\d{2}|--)
+</pre>
+it is processed as if it were
+<br>
+<br>
+(?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
+<br>
+<br>
+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
+<a href="pcretest.html"><b>pcretest</b></a>
+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.
+</P>
+<br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br>
+<P>
+You should be aware that, because of optimizations in the way PCRE matches
+patterns by default, callouts sometimes do not happen. For example, if the
+pattern is
+<pre>
+ ab(?C4)cd
+</pre>
+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.
+</P>
+<P>
+If the pattern is studied, PCRE knows the minimum length of a matching string,
+and will immediately give a "no match" return without actually running a match
+if the subject is not long enough, or, for unanchored patterns, if it has
+been scanned far enough.
+</P>
+<P>
+You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
+option to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. This slows down the
+matching process, but does ensure that callouts such as the example above are
+obeyed.
+</P>
+<br><a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a><br>
+<P>
+During matching, when PCRE reaches a callout point, the external function
+defined by <i>pcre_callout</i> is called (if it is set). This applies to both
+the <b>pcre_exec()</b> and the <b>pcre_dfa_exec()</b> matching functions. The
+only argument to the callout function is a pointer to a <b>pcre_callout</b>
+block. This structure contains the following fields:
+<pre>
+ int <i>version</i>;
+ int <i>callout_number</i>;
+ int *<i>offset_vector</i>;
+ const char *<i>subject</i>;
+ int <i>subject_length</i>;
+ int <i>start_match</i>;
+ int <i>current_position</i>;
+ int <i>capture_top</i>;
+ int <i>capture_last</i>;
+ void *<i>callout_data</i>;
+ int <i>pattern_position</i>;
+ int <i>next_item_length</i>;
+</pre>
+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>
+<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>
+<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> or <b>pcre_dfa_exec()</b>. When
+<b>pcre_exec()</b> is used, 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. For <b>pcre_dfa_exec()</b> this field is
+not useful.
+</P>
+<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>
+<P>
+The <i>start_match</i> field normally contains the offset within the subject at
+which the current match attempt started. However, if the escape sequence \K
+has been encountered, this value is changed to reflect the modified starting
+point. If the pattern is not anchored, the callout function may be called
+several times from the same point in the pattern for different starting points
+in the subject.
+</P>
+<P>
+The <i>current_position</i> field contains the offset within the subject of the
+current match pointer.
+</P>
+<P>
+When the <b>pcre_exec()</b> function is used, 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. This is always the case when <b>pcre_dfa_exec()</b> is used, because it
+does not support captured substrings.
+</P>
+<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. This is always
+the case when <b>pcre_dfa_exec()</b> is used.
+</P>
+<P>
+The <i>callout_data</i> field contains a value that is passed to
+<b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> 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
+<a href="pcreapi.html"><b>pcreapi</b></a>
+documentation.
+</P>
+<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>
+<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>
+<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.
+</P>
+<br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
+<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 the testing of other matching possibilities goes
+ahead, just as if a lookahead assertion had failed. If the value is less than
+zero, the match is abandoned, and <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>
+returns the negative value.
+</P>
+<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>
+<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
+<P>
+Philip Hazel
+<br>
+University Computing Service
+<br>
+Cambridge CB2 3QH, England.
+<br>
+</P>
+<br><a name="SEC6" href="#TOC1">REVISION</a><br>
+<P>
+Last updated: 29 September 2009
+<br>
+Copyright © 1997-2009 University of Cambridge.
+<br>
+<p>
+Return to the <a href="index.html">PCRE index page</a>.
+</p>
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