| .TH PCRECALLOUT 3 |
| .SH NAME |
| PCRE - Perl-compatible regular expressions |
| .SH "PCRE CALLOUTS" |
| .rs |
| .sp |
| .B int (*pcre_callout)(pcre_callout_block *); |
| .PP |
| 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 \fIpcre_callout\fP. By default, this variable contains NULL, |
| which disables all calling out. |
| .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: |
| .sp |
| (?C1)abc(?C2)def |
| .sp |
| If the PCRE_AUTO_CALLOUT option bit is set when \fBpcre_compile()\fP or |
| \fBpcre_compile2()\fP 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 |
| .sp |
| A(\ed{2}|--) |
| .sp |
| it is processed as if it were |
| .sp |
| (?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255) |
| .sp |
| 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 |
| .\" HREF |
| \fBpcretest\fP |
| .\" |
| 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 |
| 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. |
| . |
| . |
| .SH "MISSING CALLOUTS" |
| .rs |
| .sp |
| 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 |
| .sp |
| ab(?C4)cd |
| .sp |
| 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 |
| 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 |
| You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE |
| option to \fBpcre_compile()\fP, \fBpcre_exec()\fP, or \fBpcre_dfa_exec()\fP, |
| 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. |
| . |
| . |
| .SH "THE CALLOUT INTERFACE" |
| .rs |
| .sp |
| During matching, when PCRE reaches a callout point, the external function |
| defined by \fIpcre_callout\fP is called (if it is set). This applies to both |
| the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The |
| only argument to the callout function is a pointer to a \fBpcre_callout\fP |
| block. This structure contains the following fields: |
| .sp |
| int \fIversion\fP; |
| int \fIcallout_number\fP; |
| int *\fIoffset_vector\fP; |
| const char *\fIsubject\fP; |
| int \fIsubject_length\fP; |
| int \fIstart_match\fP; |
| int \fIcurrent_position\fP; |
| int \fIcapture_top\fP; |
| int \fIcapture_last\fP; |
| void *\fIcallout_data\fP; |
| int \fIpattern_position\fP; |
| int \fInext_item_length\fP; |
| const unsigned char *\fImark\fP; |
| .sp |
| The \fIversion\fP 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 |
| The \fIcallout_number\fP 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 |
| The \fIoffset_vector\fP field is a pointer to the vector of offsets that was |
| passed by the caller to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When |
| \fBpcre_exec()\fP 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 \fBpcre_dfa_exec()\fP this field is |
| not useful. |
| .P |
| The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values |
| that were passed to \fBpcre_exec()\fP. |
| .P |
| The \fIstart_match\fP field normally contains the offset within the subject at |
| which the current match attempt started. However, if the escape sequence \eK |
| 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 |
| The \fIcurrent_position\fP field contains the offset within the subject of the |
| current match pointer. |
| .P |
| When the \fBpcre_exec()\fP function is used, the \fIcapture_top\fP field |
| contains one more than the number of the highest numbered captured substring so |
| far. If no substrings have been captured, the value of \fIcapture_top\fP is |
| one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it |
| does not support captured substrings. |
| .P |
| The \fIcapture_last\fP 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 \fBpcre_dfa_exec()\fP is used. |
| .P |
| The \fIcallout_data\fP field contains a value that is passed to |
| \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be |
| passed back in callouts. It is passed in the \fIpcre_callout\fP field of the |
| \fBpcre_extra\fP data structure. If no such data was passed, the value of |
| \fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a |
| description of the \fBpcre_extra\fP structure in the |
| .\" HREF |
| \fBpcreapi\fP |
| .\" |
| documentation. |
| .P |
| The \fIpattern_position\fP field is present from version 1 of the |
| \fIpcre_callout\fP structure. It contains the offset to the next item to be |
| matched in the pattern string. |
| .P |
| The \fInext_item_length\fP field is present from version 1 of the |
| \fIpcre_callout\fP 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 |
| The \fIpattern_position\fP and \fInext_item_length\fP 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 |
| The \fImark\fP field is present from version 2 of the \fIpcre_callout\fP |
| structure. In callouts from \fBpcre_exec()\fP 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 \fBpcre_dfa_exec()\fP this field always contains NULL. |
| . |
| . |
| .SH "RETURN VALUES" |
| .rs |
| .sp |
| 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 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP |
| returns the negative value. |
| .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. |
| . |
| . |
| .SH AUTHOR |
| .rs |
| .sp |
| .nf |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| .fi |
| . |
| . |
| .SH REVISION |
| .rs |
| .sp |
| .nf |
| Last updated: 30 November 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| .fi |