pcrecallout(3) — Linux manual page


PCRECALLOUT(3)          Library Functions Manual          PCRECALLOUT(3)

NAME         top

       PCRE - Perl-compatible regular expressions

SYNOPSIS         top

       #include <pcre.h>

       int (*pcre_callout)(pcre_callout_block *);

       int (*pcre16_callout)(pcre16_callout_block *);

       int (*pcre32_callout)(pcre32_callout_block *);

DESCRIPTION         top

       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
       pcre_callout (pcre16_callout for the 16-bit library,
       pcre32_callout for the 32-bit library). By default, this variable
       contains NULL, which disables all calling out.

       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:


       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


       it is processed as if it were


       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:


       This applies only to assertion conditions (because they are
       themselves independent groups).

       Automatic callouts can be used for tracking the progress of
       pattern matching.  The pcretest 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.


       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.

       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 pcretest
       output when this pattern is anchored and then applied with
       automatic callouts to the string "aaaa" is:

          +0 ^        ^
          +1 ^        a+
          +3 ^   ^    [bc]
         No match

       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
       pcre_compile(), or starting the pattern with (*NO_AUTO_POSSESS).
       If this is done in pcretest (using the /O qualifier), the output
       changes to this:

          +0 ^        ^
          +1 ^        a+
          +3 ^   ^    [bc]
          +3 ^  ^     [bc]
          +3 ^ ^      [bc]
          +3 ^^       [bc]
         No match

       This time, when matching [bc] fails, the matcher backtracks into
       a+ and tries again, repeatedly, until a+ itself fails.

       Other optimizations that provide fast "no match" results also
       affect callouts.  For example, if the pattern is


       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.

       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

       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.


       During matching, when PCRE reaches a callout point, the external
       function defined by pcre_callout or pcre[16|32]_callout 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
       pcre_callout or pcre[16|32]_callout block. These structures
       contains the following fields:

         int           version;
         int           callout_number;
         int          *offset_vector;
         const char   *subject;           (8-bit version)
         PCRE_SPTR16   subject;           (16-bit version)
         PCRE_SPTR32   subject;           (32-bit version)
         int           subject_length;
         int           start_match;
         int           current_position;
         int           capture_top;
         int           capture_last;
         void         *callout_data;
         int           pattern_position;
         int           next_item_length;
         const unsigned char *mark;       (8-bit version)
         const PCRE_UCHAR16  *mark;       (16-bit version)
         const PCRE_UCHAR32  *mark;       (32-bit version)

       The version 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.

       The callout_number 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).

       The offset_vector field is a pointer to the vector of offsets
       that was passed by the caller to the matching function. When
       pcre_exec() or pcre[16|32]_exec() 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.

       The subject and subject_length fields contain copies of the
       values that were passed to the matching function.

       The start_match 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.

       The current_position field contains the offset within the subject
       of the current match pointer.

       When the pcre_exec() or pcre[16|32]_exec() is used, the
       capture_top field contains one more than the number of the
       highest numbered captured substring so far. If no substrings have
       been captured, the value of capture_top is one. This is always
       the case when the DFA functions are used, because they do not
       support captured substrings.

       The capture_last 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 capture_last is -1. This is always the case for the DFA
       matching functions.

       The callout_data 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 callout_data field of a pcre_extra
       or pcre[16|32]_extra data structure. If no such data was passed,
       the value of callout_data in a callout block is NULL. There is a
       description of the pcre_extra structure in the pcreapi

       The pattern_position 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.

       The next_item_length 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.

       The pattern_position and next_item_length 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.

       The mark field is present from version 2 of the callout
       structure. In callouts from pcre_exec() or pcre[16|32]_exec() 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.

RETURN VALUES         top

       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.

       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.

AUTHOR         top

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION         top

       Last updated: 12 November 2013
       Copyright (c) 1997-2013 University of Cambridge.

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular
       Expressions) project.  Information about the project can be found
       at ⟨http://www.pcre.org/⟩.  If you have a bug report for this
       manual page, see
       ⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩.  This page was
       obtained from the tarball pcre-8.45.tar.gz fetched from
       ⟨ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/⟩ on
       2021-08-27.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to

PCRE 8.34                   12 November 2013              PCRECALLOUT(3)

Pages that refer to this page: pcretest(1)pcreapi(3)pcrepattern(3)pcresyntax(3)