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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">PCRE CALLOUTS</a> |
| <li><a name="TOC2" href="#SEC2">MISSING CALLOUTS</a> |
| <li><a name="TOC3" href="#SEC3">THE CALLOUT INTERFACE</a> |
| <li><a name="TOC4" href="#SEC4">RETURN VALUES</a> |
| <li><a name="TOC5" href="#SEC5">AUTHOR</a> |
| <li><a name="TOC6" href="#SEC6">REVISION</a> |
| </ul> |
| <br><a name="SEC1" href="#TOC1">PCRE CALLOUTS</a><br> |
| <P> |
| <b>int (*pcre_callout)(pcre_callout_block *);</b> |
| </P> |
| <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>. 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 <b>pcre_compile()</b> or |
| <b>pcre_compile2()</b> is called, 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. Automatic callouts can be used for tracking the progress of |
| pattern matching. The |
| <a href="pcretest.html"><b>pcretest</b></a> |
| command has an option that sets automatic callouts; when it is used, the output |
| indicates how the pattern is matched. This is useful information when you are |
| trying to optimize the performance of a particular pattern. |
| </P> |
| <P> |
| The use of callouts in a pattern makes it ineligible for optimization by the |
| just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE |
| option always fails. |
| </P> |
| <br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br> |
| <P> |
| You should be aware that, because of optimizations in the way PCRE matches |
| patterns by default, callouts sometimes do not happen. 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 <b>pcre_compile()</b>, <b>pcre_exec()</b>, or <b>pcre_dfa_exec()</b>, |
| 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="SEC3" 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> is called (if it is set). This applies to both |
| the <b>pcre_exec()</b> and the <b>pcre_dfa_exec()</b> matching functions. The |
| only argument to the callout function is a pointer to a <b>pcre_callout</b> |
| block. This structure contains the following fields: |
| <pre> |
| int <i>version</i>; |
| int <i>callout_number</i>; |
| int *<i>offset_vector</i>; |
| const char *<i>subject</i>; |
| 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>; |
| </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 <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. When |
| <b>pcre_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 <b>pcre_dfa_exec()</b> 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 <b>pcre_exec()</b>. |
| </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> function 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 <b>pcre_dfa_exec()</b> is used, because it |
| does not support captured substrings. |
| </P> |
| <P> |
| The <i>capture_last</i> field contains the number of the most recently captured |
| substring. If no substrings have been captured, its value is -1. This is always |
| the case when <b>pcre_dfa_exec()</b> is used. |
| </P> |
| <P> |
| The <i>callout_data</i> field contains a value that is passed to |
| <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> specifically so that it can be |
| passed back in callouts. It is passed in the <i>pcre_callout</i> field of the |
| <b>pcre_extra</b> data structure. If no such data was passed, the value of |
| <i>callout_data</i> in a <b>pcre_callout</b> 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 |
| <i>pcre_callout</i> 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 |
| <i>pcre_callout</i> 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 <i>pcre_callout</i> |
| structure. In callouts from <b>pcre_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 <b>pcre_dfa_exec()</b> this field always contains NULL. |
| </P> |
| <br><a name="SEC4" 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, and <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> |
| 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="SEC5" href="#TOC1">AUTHOR</a><br> |
| <P> |
| Philip Hazel |
| <br> |
| University Computing Service |
| <br> |
| Cambridge CB2 3QH, England. |
| <br> |
| </P> |
| <br><a name="SEC6" href="#TOC1">REVISION</a><br> |
| <P> |
| Last updated: 30 November 2011 |
| <br> |
| Copyright © 1997-2011 University of Cambridge. |
| <br> |
| <p> |
| Return to the <a href="index.html">PCRE index page</a>. |
| </p> |