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| <title>pcrecallout specification</title> |
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| <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">SYNOPSIS</a> |
| <li><a name="TOC2" href="#SEC2">DESCRIPTION</a> |
| <li><a name="TOC3" href="#SEC3">MISSING CALLOUTS</a> |
| <li><a name="TOC4" href="#SEC4">THE CALLOUT INTERFACE</a> |
| <li><a name="TOC5" href="#SEC5">RETURN VALUES</a> |
| <li><a name="TOC6" href="#SEC6">AUTHOR</a> |
| <li><a name="TOC7" href="#SEC7">REVISION</a> |
| </ul> |
| <br><a name="SEC1" href="#TOC1">SYNOPSIS</a><br> |
| <P> |
| <b>#include <pcre.h></b> |
| </P> |
| <P> |
| <b>int (*pcre_callout)(pcre_callout_block *);</b> |
| </P> |
| <P> |
| <b>int (*pcre16_callout)(pcre16_callout_block *);</b> |
| </P> |
| <P> |
| <b>int (*pcre32_callout)(pcre32_callout_block *);</b> |
| </P> |
| <br><a name="SEC2" href="#TOC1">DESCRIPTION</a><br> |
| <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> (<i>pcre16_callout</i> for the 16-bit |
| library, <i>pcre32_callout</i> for the 32-bit library). 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 a pattern is compiled, 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. If the pattern contains a conditional group whose condition is |
| an assertion, an automatic callout is inserted immediately before the |
| condition. Such a callout may also be inserted explicitly, for example: |
| <pre> |
| (?(?C9)(?=a)ab|de) |
| </pre> |
| This applies only to assertion conditions (because they are themselves |
| independent groups). |
| </P> |
| <P> |
| Automatic callouts can be used for tracking the progress of pattern matching. |
| The |
| <a href="pcretest.html"><b>pcretest</b></a> |
| program has a pattern qualifier (/C) that sets automatic callouts; when it is |
| used, the output indicates how the pattern is being matched. This is useful |
| information when you are trying to optimize the performance of a particular |
| pattern. |
| </P> |
| <br><a name="SEC3" href="#TOC1">MISSING CALLOUTS</a><br> |
| <P> |
| You should be aware that, because of optimizations in the way PCRE compiles and |
| matches patterns, callouts sometimes do not happen exactly as you might expect. |
| </P> |
| <P> |
| At compile time, PCRE "auto-possessifies" repeated items when it knows that |
| what follows cannot be part of the repeat. For example, a+[bc] is compiled as |
| if it were a++[bc]. The <b>pcretest</b> output when this pattern is anchored and |
| then applied with automatic callouts to the string "aaaa" is: |
| <pre> |
| --->aaaa |
| +0 ^ ^ |
| +1 ^ a+ |
| +3 ^ ^ [bc] |
| No match |
| </pre> |
| This indicates that when matching [bc] fails, there is no backtracking into a+ |
| and therefore the callouts that would be taken for the backtracks do not occur. |
| You can disable the auto-possessify feature by passing PCRE_NO_AUTO_POSSESS |
| to <b>pcre_compile()</b>, or starting the pattern with (*NO_AUTO_POSSESS). If |
| this is done in <b>pcretest</b> (using the /O qualifier), the output changes to |
| this: |
| <pre> |
| --->aaaa |
| +0 ^ ^ |
| +1 ^ a+ |
| +3 ^ ^ [bc] |
| +3 ^ ^ [bc] |
| +3 ^ ^ [bc] |
| +3 ^^ [bc] |
| No match |
| </pre> |
| This time, when matching [bc] fails, the matcher backtracks into a+ and tries |
| again, repeatedly, until a+ itself fails. |
| </P> |
| <P> |
| Other optimizations that provide fast "no match" results also affect callouts. |
| 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 the matching function, or by starting the pattern with |
| (*NO_START_OPT). This slows down the matching process, but does ensure that |
| callouts such as the example above are obeyed. |
| </P> |
| <br><a name="SEC4" 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> or <i>pcre[16|32]_callout</i> is called (if it is |
| set). This applies to both normal and DFA matching. The only argument to the |
| callout function is a pointer to a <b>pcre_callout</b> or |
| <b>pcre[16|32]_callout</b> block. These structures contains the following |
| fields: |
| <pre> |
| int <i>version</i>; |
| int <i>callout_number</i>; |
| int *<i>offset_vector</i>; |
| const char *<i>subject</i>; (8-bit version) |
| PCRE_SPTR16 <i>subject</i>; (16-bit version) |
| PCRE_SPTR32 <i>subject</i>; (32-bit version) |
| 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>; |
| const unsigned char *<i>mark</i>; (8-bit version) |
| const PCRE_UCHAR16 *<i>mark</i>; (16-bit version) |
| const PCRE_UCHAR32 *<i>mark</i>; (32-bit version) |
| </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 2. 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 the matching function. When <b>pcre_exec()</b> or |
| <b>pcre[16|32]_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 the DFA matching |
| functions, 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 the matching function. |
| </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> or <b>pcre[16|32]_exec()</b> 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 the DFA |
| functions are used, because they do not support captured substrings. |
| </P> |
| <P> |
| The <i>capture_last</i> field contains the number of the most recently captured |
| substring. However, when a recursion exits, the value reverts to what it was |
| outside the recursion, as do the values of all captured substrings. If no |
| substrings have been captured, the value of <i>capture_last</i> is -1. This is |
| always the case for the DFA matching functions. |
| </P> |
| <P> |
| The <i>callout_data</i> field contains a value that is passed to a matching |
| function specifically so that it can be passed back in callouts. It is passed |
| in the <i>callout_data</i> field of a <b>pcre_extra</b> or <b>pcre[16|32]_extra</b> |
| data structure. If no such data was passed, the value of <i>callout_data</i> in |
| a callout 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 callout |
| 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 callout |
| 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> |
| <P> |
| The <i>mark</i> field is present from version 2 of the callout structure. In |
| callouts from <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b> it contains a |
| pointer to the zero-terminated name of the most recently passed (*MARK), |
| (*PRUNE), or (*THEN) item in the match, or NULL if no such items have been |
| passed. Instances of (*PRUNE) or (*THEN) without a name do not obliterate a |
| previous (*MARK). In callouts from the DFA matching functions this field always |
| contains NULL. |
| </P> |
| <br><a name="SEC5" 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, the matching function 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="SEC6" href="#TOC1">AUTHOR</a><br> |
| <P> |
| Philip Hazel |
| <br> |
| University Computing Service |
| <br> |
| Cambridge CB2 3QH, England. |
| <br> |
| </P> |
| <br><a name="SEC7" href="#TOC1">REVISION</a><br> |
| <P> |
| Last updated: 12 November 2013 |
| <br> |
| Copyright © 1997-2013 University of Cambridge. |
| <br> |
| <p> |
| Return to the <a href="index.html">PCRE index page</a>. |
| </p> |
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