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libraries/spcre/libpcre/pcre/doc/html/pcrecallout.html
author Tom Sutcliffe <thomas.sutcliffe@accenture.com>
Sat, 31 Jul 2010 19:07:57 +0100
changeset 23 092bcc217d9d
parent 0 7f656887cf89
permissions -rw-r--r--
Tidied iocli exports, build macro tweaks. Removed 4 overloads of CCommandBase::RunCommand[L] that are no longer used at all, and changed one more to not be exported as it's only used internally to iocli.dll. fixed builds on platforms that don't support btrace or any form of tracing.
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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