| .TH PCRECALLOUT 3 "12 November 2013" "PCRE 8.34" |
| .SH NAME |
| PCRE - Perl-compatible regular expressions |
| .SH SYNOPSIS |
| .rs |
| .sp |
| .B #include <pcre.h> |
| .PP |
| .SM |
| .B int (*pcre_callout)(pcre_callout_block *); |
| .PP |
| .B int (*pcre16_callout)(pcre16_callout_block *); |
| .PP |
| .B int (*pcre32_callout)(pcre32_callout_block *); |
| . |
| .SH DESCRIPTION |
| .rs |
| .sp |
| 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 (\fIpcre16_callout\fP for the 16-bit |
| library, \fIpcre32_callout\fP for the 32-bit library). 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 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 |
| .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. 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: |
| .sp |
| (?(?C9)(?=a)ab|de) |
| .sp |
| This applies only to assertion conditions (because they are themselves |
| independent groups). |
| .P |
| Automatic callouts can be used for tracking the progress of pattern matching. |
| The |
| .\" HREF |
| \fBpcretest\fP |
| .\" |
| 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. |
| . |
| . |
| .SH "MISSING CALLOUTS" |
| .rs |
| .sp |
| 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 |
| 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 \fBpcretest\fP output when this pattern is anchored and |
| then applied with automatic callouts to the string "aaaa" is: |
| .sp |
| --->aaaa |
| +0 ^ ^ |
| +1 ^ a+ |
| +3 ^ ^ [bc] |
| No match |
| .sp |
| 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 \fBpcre_compile()\fP, or starting the pattern with (*NO_AUTO_POSSESS). If |
| this is done in \fBpcretest\fP (using the /O qualifier), the output changes to |
| this: |
| .sp |
| --->aaaa |
| +0 ^ ^ |
| +1 ^ a+ |
| +3 ^ ^ [bc] |
| +3 ^ ^ [bc] |
| +3 ^ ^ [bc] |
| +3 ^^ [bc] |
| No match |
| .sp |
| This time, when matching [bc] fails, the matcher backtracks into a+ and tries |
| again, repeatedly, until a+ itself fails. |
| .P |
| Other optimizations that provide fast "no match" results also affect callouts. |
| 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 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. |
| . |
| . |
| .SH "THE CALLOUT INTERFACE" |
| .rs |
| .sp |
| During matching, when PCRE reaches a callout point, the external function |
| defined by \fIpcre_callout\fP or \fIpcre[16|32]_callout\fP 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 \fBpcre_callout\fP or |
| \fBpcre[16|32]_callout\fP block. These structures contains the following |
| fields: |
| .sp |
| int \fIversion\fP; |
| int \fIcallout_number\fP; |
| int *\fIoffset_vector\fP; |
| const char *\fIsubject\fP; (8-bit version) |
| PCRE_SPTR16 \fIsubject\fP; (16-bit version) |
| PCRE_SPTR32 \fIsubject\fP; (32-bit version) |
| 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; (8-bit version) |
| const PCRE_UCHAR16 *\fImark\fP; (16-bit version) |
| const PCRE_UCHAR32 *\fImark\fP; (32-bit version) |
| .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 the matching function. When \fBpcre_exec()\fP or |
| \fBpcre[16|32]_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 the DFA matching |
| functions, this field is not useful. |
| .P |
| The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values |
| that were passed to the matching function. |
| .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 or \fBpcre[16|32]_exec()\fP 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 the DFA |
| functions are used, because they do not support captured substrings. |
| .P |
| The \fIcapture_last\fP 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 \fIcapture_last\fP is -1. This is |
| always the case for the DFA matching functions. |
| .P |
| The \fIcallout_data\fP 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 \fIcallout_data\fP field of a \fBpcre_extra\fP or \fBpcre[16|32]_extra\fP |
| data structure. If no such data was passed, the value of \fIcallout_data\fP in |
| a callout 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 callout |
| 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 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 |
| 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 callout structure. In |
| callouts from \fBpcre_exec()\fP or \fBpcre[16|32]_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 the DFA matching functions 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, the matching function 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: 12 November 2013 |
| Copyright (c) 1997-2013 University of Cambridge. |
| .fi |