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<title>pcrecallout specification</title>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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This HTML document has been generated automatically from the original man page.
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If there is any nonsense in it, please consult the man page, in case the
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conversion went wrong.<br>
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<ul>
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<li><a name="TOC1" href="#SEC1">PCRE CALLOUTS</a>
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<li><a name="TOC2" href="#SEC2">RETURN VALUES</a>
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</ul>
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<br><a name="SEC1" href="#TOC1">PCRE CALLOUTS</a><br>
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<P>
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<b>int (*pcre_callout)(pcre_callout_block *);</b>
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</P>
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<P>
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PCRE provides a feature called "callout", which is a means of temporarily
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passing control to the caller of PCRE in the middle of pattern matching. The
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caller of PCRE provides an external function by putting its entry point in the
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global variable <i>pcre_callout</i>. By default, this variable contains NULL,
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which disables all calling out.
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</P>
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<P>
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Within a regular expression, (?C) indicates the points at which the external
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function is to be called. Different callout points can be identified by putting
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a number less than 256 after the letter C. The default value is zero.
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For example, this pattern has two callout points:
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</P>
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<P>
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<pre>
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(?C1)\dabc(?C2)def
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</PRE>
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</P>
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<P>
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During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
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set), the external function is called. Its only argument is a pointer to a
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<b>pcre_callout</b> block. This contains the following variables:
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</P>
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<P>
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<pre>
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int <i>version</i>;
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int <i>callout_number</i>;
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int *<i>offset_vector</i>;
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const char *<i>subject</i>;
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int <i>subject_length</i>;
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int <i>start_match</i>;
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int <i>current_position</i>;
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int <i>capture_top</i>;
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int <i>capture_last</i>;
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void *<i>callout_data</i>;
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</PRE>
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</P>
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<P>
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The <i>version</i> field is an integer containing the version number of the
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block format. The current version is zero. The version number may change in
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future if additional fields are added, but the intention is never to remove any
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of the existing fields.
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</P>
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<P>
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The <i>callout_number</i> field contains the number of the callout, as compiled
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into the pattern (that is, the number after ?C).
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</P>
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<P>
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The <i>offset_vector</i> field is a pointer to the vector of offsets that was
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passed by the caller to <b>pcre_exec()</b>. The contents can be inspected in
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order to extract substrings that have been matched so far, in the same way as
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for extracting substrings after a match has completed.
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</P>
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<P>
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The <i>subject</i> and <i>subject_length</i> fields contain copies the values
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that were passed to <b>pcre_exec()</b>.
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</P>
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<P>
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The <i>start_match</i> field contains the offset within the subject at which the
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current match attempt started. If the pattern is not anchored, the callout
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function may be called several times for different starting points.
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</P>
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<P>
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The <i>current_position</i> field contains the offset within the subject of the
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current match pointer.
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</P>
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<P>
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The <i>capture_top</i> field contains the number of the highest captured
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substring so far.
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</P>
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<P>
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The <i>capture_last</i> field contains the number of the most recently captured
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substring.
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</P>
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<P>
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The <i>callout_data</i> field contains a value that is passed to
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<b>pcre_exec()</b> by the caller specifically so that it can be passed back in
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callouts. It is passed in the <i>pcre_callout</i> field of the <b>pcre_extra</b>
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data structure. If no such data was passed, the value of <i>callout_data</i> in
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a <b>pcre_callout</b> block is NULL. There is a description of the
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<b>pcre_extra</b> structure in the <b>pcreapi</b> documentation.
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</P>
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<br><a name="SEC2" href="#TOC1">RETURN VALUES</a><br>
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<P>
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The callout function returns an integer. If the value is zero, matching
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proceeds as normal. If the value is greater than zero, matching fails at the
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current point, but backtracking to test other possibilities goes ahead, just as
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if a lookahead assertion had failed. If the value is less than zero, the match
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is abandoned, and <b>pcre_exec()</b> returns the value.
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</P>
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<P>
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Negative values should normally be chosen from the set of PCRE_ERROR_xxx
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values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
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The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
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it will never be used by PCRE itself.
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</P>
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<P>
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Last updated: 21 January 2003
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<br>
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Copyright © 1997-2003 University of Cambridge.
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