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libraries/spcre/libpcre/pcre/doc/html/pcrecallout.html
author Tom Sutcliffe <thomas.sutcliffe@accenture.com>
Wed, 23 Jun 2010 15:52:26 +0100
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<html>
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<head>
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<title>pcrecallout specification</title>
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</head>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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<h1>pcrecallout man page</h1>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
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<p>
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This page is part of the PCRE HTML documentation. It was generated automatically
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from the original man page. If there is any nonsense in it, please consult the
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man page, in case the conversion went wrong.
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<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">MISSING CALLOUTS</a>
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<li><a name="TOC3" href="#SEC3">THE CALLOUT INTERFACE</a>
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<li><a name="TOC4" href="#SEC4">RETURN VALUES</a>
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<li><a name="TOC5" href="#SEC5">AUTHOR</a>
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<li><a name="TOC6" href="#SEC6">REVISION</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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<pre>
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  (?C1)abc(?C2)def
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</pre>
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If the PCRE_AUTO_CALLOUT option bit is set when <b>pcre_compile()</b> is called,
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PCRE automatically inserts callouts, all with number 255, before each item in
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the pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
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<pre>
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  A(\d{2}|--)
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</pre>
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it is processed as if it were
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<br>
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<br>
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(?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
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<br>
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<br>
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Notice that there is a callout before and after each parenthesis and
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alternation bar. Automatic callouts can be used for tracking the progress of
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pattern matching. The
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<a href="pcretest.html"><b>pcretest</b></a>
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command has an option that sets automatic callouts; when it is used, the output
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indicates how the pattern is matched. This is useful information when you are
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trying to optimize the performance of a particular pattern.
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</P>
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<br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br>
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<P>
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You should be aware that, because of optimizations in the way PCRE matches
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patterns, callouts sometimes do not happen. For example, if the pattern is
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<pre>
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  ab(?C4)cd
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</pre>
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PCRE knows that any matching string must contain the letter "d". If the subject
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string is "abyz", the lack of "d" means that matching doesn't ever start, and
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the callout is never reached. However, with "abyd", though the result is still
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no match, the callout is obeyed.
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</P>
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<br><a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a><br>
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<P>
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During matching, when PCRE reaches a callout point, the external function
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defined by <i>pcre_callout</i> is called (if it is set). This applies to both
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the <b>pcre_exec()</b> and the <b>pcre_dfa_exec()</b> matching functions. The
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only argument to the callout function is a pointer to a <b>pcre_callout</b>
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block. This structure contains the following fields:
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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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  int          <i>pattern_position</i>;
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  int          <i>next_item_length</i>;
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</pre>
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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 initial version was 0; the current version is 1. The version
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number will change again in future if additional fields are added, but the
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intention is never to remove any 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 for manual callouts, and 255 for
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automatically generated callouts).
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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> or <b>pcre_dfa_exec()</b>. When
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<b>pcre_exec()</b> is used, the contents can be inspected in order to extract
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substrings that have been matched so far, in the same way as for extracting
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substrings after a match has completed. For <b>pcre_dfa_exec()</b> this field is
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not useful.
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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 of 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 normally contains the offset within the subject at
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which the current match attempt started. However, if the escape sequence \K
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has been encountered, this value is changed to reflect the modified starting
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point. If the pattern is not anchored, the callout function may be called
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several times from the same point in the pattern for different starting points
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in the subject.
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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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When the <b>pcre_exec()</b> function is used, the <i>capture_top</i> field
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contains one more than the number of the highest numbered captured substring so
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far. If no substrings have been captured, the value of <i>capture_top</i> is
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one. This is always the case when <b>pcre_dfa_exec()</b> is used, because it
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does not support captured substrings.
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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. If no substrings have been captured, its value is -1. This is always
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the case when <b>pcre_dfa_exec()</b> is used.
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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> or <b>pcre_dfa_exec()</b> specifically so that it can be
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passed back in callouts. It is passed in the <i>pcre_callout</i> field of the
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<b>pcre_extra</b> data structure. If no such data was passed, the value of
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<i>callout_data</i> in a <b>pcre_callout</b> block is NULL. There is a
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description of the <b>pcre_extra</b> structure in the
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<a href="pcreapi.html"><b>pcreapi</b></a>
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documentation.
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</P>
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<P>
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The <i>pattern_position</i> field is present from version 1 of the
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<i>pcre_callout</i> structure. It contains the offset to the next item to be
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matched in the pattern string.
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</P>
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<P>
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The <i>next_item_length</i> field is present from version 1 of the
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<i>pcre_callout</i> structure. It contains the length of the next item to be
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matched in the pattern string. When the callout immediately precedes an
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alternation bar, a closing parenthesis, or the end of the pattern, the length
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is zero. When the callout precedes an opening parenthesis, the length is that
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of the entire subpattern.
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</P>
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<P>
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The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
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help in distinguishing between different automatic callouts, which all have the
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same callout number. However, they are set for all callouts.
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</P>
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<br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
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<P>
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The external callout function returns an integer to PCRE. If the value is zero,
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matching proceeds as normal. If the value is greater than zero, matching fails
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at the current point, but the testing of other matching possibilities goes
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ahead, just as if a lookahead assertion had failed. If the value is less than
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zero, the match is abandoned, and <b>pcre_exec()</b> (or <b>pcre_dfa_exec()</b>)
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returns the negative 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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<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
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<P>
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Philip Hazel
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<br>
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University Computing Service
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<br>
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Cambridge CB2 3QH, England.
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<br>
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</P>
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<br><a name="SEC6" href="#TOC1">REVISION</a><br>
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<P>
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Last updated: 29 May 2007
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<br>
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Copyright &copy; 1997-2007 University of Cambridge.
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<br>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
FCL/sf/os/fshell