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<head>
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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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<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">SYNOPSIS</a>
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<li><a name="TOC2" href="#SEC2">DESCRIPTION</a>
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<li><a name="TOC3" href="#SEC3">MISSING CALLOUTS</a>
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<li><a name="TOC4" href="#SEC4">THE CALLOUT INTERFACE</a>
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<li><a name="TOC5" href="#SEC5">RETURN VALUES</a>
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<li><a name="TOC6" href="#SEC6">AUTHOR</a>
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<li><a name="TOC7" href="#SEC7">REVISION</a>
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</ul>
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<br><a name="SEC1" href="#TOC1">SYNOPSIS</a><br>
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<P>
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<b>#include <pcre.h></b>
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</P>
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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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<b>int (*pcre16_callout)(pcre16_callout_block *);</b>
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</P>
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<P>
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<b>int (*pcre32_callout)(pcre32_callout_block *);</b>
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</P>
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<br><a name="SEC2" href="#TOC1">DESCRIPTION</a><br>
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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> (<i>pcre16_callout</i> for the 16-bit
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library, <i>pcre32_callout</i> for the 32-bit library). By default, this
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variable contains NULL, 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 a pattern is compiled, PCRE
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automatically inserts callouts, all with number 255, before each item in the
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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. If the pattern contains a conditional group whose condition is
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an assertion, an automatic callout is inserted immediately before the
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condition. Such a callout may also be inserted explicitly, for example:
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<pre>
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(?(?C9)(?=a)ab|de)
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</pre>
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This applies only to assertion conditions (because they are themselves
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independent groups).
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</P>
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<P>
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Automatic callouts can be used for tracking the progress of pattern matching.
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The
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<a href="pcretest.html"><b>pcretest</b></a>
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program has a pattern qualifier (/C) that sets automatic callouts; when it is
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used, the output indicates how the pattern is being matched. This is useful
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information when you are trying to optimize the performance of a particular
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pattern.
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</P>
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<br><a name="SEC3" 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 compiles and
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matches patterns, callouts sometimes do not happen exactly as you might expect.
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</P>
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<P>
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At compile time, PCRE "auto-possessifies" repeated items when it knows that
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what follows cannot be part of the repeat. For example, a+[bc] is compiled as
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if it were a++[bc]. The <b>pcretest</b> output when this pattern is anchored and
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then applied with automatic callouts to the string "aaaa" is:
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<pre>
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--->aaaa
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+0 ^ ^
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+1 ^ a+
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+3 ^ ^ [bc]
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No match
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</pre>
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This indicates that when matching [bc] fails, there is no backtracking into a+
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and therefore the callouts that would be taken for the backtracks do not occur.
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You can disable the auto-possessify feature by passing PCRE_NO_AUTO_POSSESS
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to <b>pcre_compile()</b>, or starting the pattern with (*NO_AUTO_POSSESS). If
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this is done in <b>pcretest</b> (using the /O qualifier), the output changes to
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this:
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<pre>
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--->aaaa
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+0 ^ ^
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+1 ^ a+
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+3 ^ ^ [bc]
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+3 ^ ^ [bc]
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+3 ^ ^ [bc]
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+3 ^^ [bc]
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No match
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</pre>
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This time, when matching [bc] fails, the matcher backtracks into a+ and tries
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again, repeatedly, until a+ itself fails.
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</P>
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<P>
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Other optimizations that provide fast "no match" results also affect callouts.
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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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<P>
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If the pattern is studied, PCRE knows the minimum length of a matching string,
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and will immediately give a "no match" return without actually running a match
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if the subject is not long enough, or, for unanchored patterns, if it has
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been scanned far enough.
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</P>
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<P>
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You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
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option to the matching function, or by starting the pattern with
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(*NO_START_OPT). This slows down the matching process, but does ensure that
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callouts such as the example above are obeyed.
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</P>
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<br><a name="SEC4" 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> or <i>pcre[16|32]_callout</i> is called (if it is
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set). This applies to both normal and DFA matching. The only argument to the
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callout function is a pointer to a <b>pcre_callout</b> or
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<b>pcre[16|32]_callout</b> block. These structures contains the following
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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>; (8-bit version)
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PCRE_SPTR16 <i>subject</i>; (16-bit version)
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PCRE_SPTR32 <i>subject</i>; (32-bit version)
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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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const unsigned char *<i>mark</i>; (8-bit version)
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const PCRE_UCHAR16 *<i>mark</i>; (16-bit version)
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const PCRE_UCHAR32 *<i>mark</i>; (32-bit version)
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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 2. 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 the matching function. When <b>pcre_exec()</b> or
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<b>pcre[16|32]_exec()</b> is used, the contents can be inspected, in order to
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extract substrings that have been matched so far, in the same way as for
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extracting substrings after a match has completed. For the DFA matching
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functions, this field is 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 the matching function.
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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> or <b>pcre[16|32]_exec()</b> is used, the
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<i>capture_top</i> field contains one more than the number of the highest
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numbered captured substring so far. If no substrings have been captured, the
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value of <i>capture_top</i> is one. This is always the case when the DFA
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functions are used, because they do 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. However, when a recursion exits, the value reverts to what it was
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outside the recursion, as do the values of all captured substrings. If no
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substrings have been captured, the value of <i>capture_last</i> is -1. This is
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always the case for the DFA matching functions.
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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 a matching
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function specifically so that it can be passed back in callouts. It is passed
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in the <i>callout_data</i> field of a <b>pcre_extra</b> or <b>pcre[16|32]_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 callout block is NULL. There is a description of the <b>pcre_extra</b>
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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 callout
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structure. It contains the offset to the next item to be matched in the pattern
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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 callout
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structure. It contains the length of the next item to be matched in the pattern
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string. When the callout immediately precedes an alternation bar, a closing
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parenthesis, or the end of the pattern, the length is zero. When the callout
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precedes an opening parenthesis, the length is that 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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<P>
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The <i>mark</i> field is present from version 2 of the callout structure. In
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callouts from <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b> it contains a
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pointer to the zero-terminated name of the most recently passed (*MARK),
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(*PRUNE), or (*THEN) item in the match, or NULL if no such items have been
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passed. Instances of (*PRUNE) or (*THEN) without a name do not obliterate a
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previous (*MARK). In callouts from the DFA matching functions this field always
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contains NULL.
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</P>
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<br><a name="SEC5" 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, the matching function 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="SEC6" 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="SEC7" href="#TOC1">REVISION</a><br>
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<P>
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Last updated: 12 November 2013
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<br>
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Copyright © 1997-2013 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>
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