pcre2callout(3) — Linux manual page


PCRE2CALLOUT(3)         Library Functions Manual         PCRE2CALLOUT(3)

NAME         top

       PCRE2 - Perl-compatible regular expressions (revised API)

SYNOPSIS         top

       #include <pcre2.h>

       int (*pcre2_callout)(pcre2_callout_block *, void *);

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);

DESCRIPTION         top

       PCRE2 provides a feature called "callout", which is a means of
       temporarily passing control to the caller of PCRE2 in the middle
       of pattern matching. The caller of PCRE2 provides an external
       function by putting its entry point in a match context (see
       pcre2_set_callout() in the pcre2api documentation).

       When using the pcre2_substitute() function, an additional callout
       feature is available. This does a callout after each change to
       the subject string and is described in the pcre2api
       documentation; the rest of this document is concerned with
       callouts during pattern matching.

       Within a regular expression, (?C<arg>) indicates a point 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.  Alternatively, the argument
       may be a delimited string. The starting delimiter must be one of
       ` ' " ^ % # $ { and the ending delimiter is the same as the
       start, except for {, where the ending delimiter is }. If the
       ending delimiter is needed within the string, it must be doubled.
       For example, this pattern has two callout points:

         (?C1)abc(?C"some ""arbitrary"" text")def

       If the PCRE2_AUTO_CALLOUT option bit is set when a pattern is
       compiled, PCRE2 automatically inserts callouts, all with number
       255, before each item in the pattern except for immediately
       before or after an explicit callout. For example, if
       PCRE2_AUTO_CALLOUT is used with the pattern


       it is processed as if it were


       Here is a more complicated example:


       With PCRE2_AUTO_CALLOUT, this pattern 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:

         (?(?C9)(?=a)ab|de)  (?(?C%text%)(?!=d)ab|de)

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

       Callouts can be useful for tracking the progress of pattern
       matching. The pcre2test program has a pattern qualifier
       (/auto_callout) that sets automatic callouts.  When any callouts
       are present, the output from pcre2test 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
       PCRE2 compiles and matches patterns, callouts sometimes do not
       happen exactly as you might expect.


       At compile time, PCRE2 "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 pcre2test
       output when this pattern is compiled with PCRE2_ANCHORED and
       PCRE2_AUTO_CALLOUT and then applied to the string "aaaa" is:

          +0 ^        a+
          +2 ^   ^    [bc]
         No match

       This indicates that when matching [bc] fails, there is no
       backtracking into a+ (because it is being treated as a++) and
       therefore the callouts that would be taken for the backtracks do
       not occur. You can disable the auto-possessify feature by passing
       PCRE2_NO_AUTO_POSSESS to pcre2_compile(), or starting the pattern
       with (*NO_AUTO_POSSESS). In this case, the output changes to

          +0 ^        a+
          +2 ^   ^    [bc]
          +2 ^  ^     [bc]
          +2 ^ ^      [bc]
          +2 ^^       [bc]
         No match

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

   Automatic .* anchoring

       By default, an optimization is applied when .* is the first
       significant item in a pattern. If PCRE2_DOTALL is set, so that
       the dot can match any character, the pattern is automatically
       anchored. If PCRE2_DOTALL is not set, a match can start only
       after an internal newline or at the beginning of the subject, and
       pcre2_compile() remembers this. If a pattern has more than one
       top-level branch, automatic anchoring occurs if all branches are

       This optimization is disabled, however, if .* is in an atomic
       group or if there is a backreference to the capture group in
       which it appears. It is also disabled if the pattern contains
       (*PRUNE) or (*SKIP). However, the presence of callouts does not
       affect it.

       For example, if the pattern .*\d is compiled with
       PCRE2_AUTO_CALLOUT and applied to the string "aa", the pcre2test
       output is:

          +0 ^      .*
          +2 ^ ^    \d
          +2 ^^     \d
          +2 ^      \d
         No match

       This shows that all match attempts start at the beginning of the
       subject. In other words, the pattern is anchored. You can disable
       this optimization by passing PCRE2_NO_DOTSTAR_ANCHOR to
       pcre2_compile(), or starting the pattern with
       (*NO_DOTSTAR_ANCHOR). In this case, the output changes to:

          +0 ^      .*
          +2 ^ ^    \d
          +2 ^^     \d
          +2 ^      \d
          +0  ^     .*
          +2  ^^    \d
          +2  ^     \d
         No match

       This shows more match attempts, starting at the second subject
       character.  Another optimization, described in the next section,
       means that there is no subsequent attempt to match with an empty

   Other optimizations

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


       PCRE2 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.

       For most patterns PCRE2 also 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
       PCRE2_NO_START_OPTIMIZE option to pcre2_compile(), 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 PCRE2 reaches a callout point, if an
       external function is provided in the match context, it is called.
       This applies to both normal, DFA, and JIT matching. The first
       argument to the callout function is a pointer to a pcre2_callout
       block. The second argument is the void * callout data that was
       supplied when the callout was set up by calling
       pcre2_set_callout() (see the pcre2api documentation). The callout
       block structure contains the following fields, not necessarily in
       this order:

         uint32_t      version;
         uint32_t      callout_number;
         uint32_t      capture_top;
         uint32_t      capture_last;
         uint32_t      callout_flags;
         PCRE2_SIZE   *offset_vector;
         PCRE2_SPTR    mark;
         PCRE2_SPTR    subject;
         PCRE2_SIZE    subject_length;
         PCRE2_SIZE    start_match;
         PCRE2_SIZE    current_position;
         PCRE2_SIZE    pattern_position;
         PCRE2_SIZE    next_item_length;
         PCRE2_SIZE    callout_string_offset;
         PCRE2_SIZE    callout_string_length;
         PCRE2_SPTR    callout_string;

       The version field contains the version number of the block
       format. The current version is 2; the three callout string fields
       were added for version 1, and the callout_flags field for version
       2. If you are writing an application that might use an earlier
       release of PCRE2, you should check the version number before
       accessing any of these fields. The version number will increase
       in future if more fields are added, but the intention is never to
       remove any of the existing fields.

   Fields for numerical callouts

       For a numerical callout, callout_string is NULL, and
       callout_number contains the number of the callout, in the range
       0-255. This is the number that follows (?C for callouts that part
       of the pattern; it is 255 for automatically generated callouts.

   Fields for string callouts

       For callouts with string arguments, callout_number is always
       zero, and callout_string points to the string that is contained
       within the compiled pattern. Its length is given by
       callout_string_length. Duplicated ending delimiters that were
       present in the original pattern string have been turned into
       single characters, but there is no other processing of the
       callout string argument. An additional code unit containing
       binary zero is present after the string, but is not included in
       the length. The delimiter that was used to start the string is
       also stored within the pattern, immediately before the string
       itself. You can access this delimiter as callout_string[-1] if
       you need it.

       The callout_string_offset field is the code unit offset to the
       start of the callout argument string within the original pattern
       string. This is provided for the benefit of applications such as
       script languages that might need to report errors in the callout
       string within the pattern.

   Fields for all callouts

       The remaining fields in the callout block are the same for both
       kinds of callout.

       The offset_vector field is a pointer to a vector of capturing
       offsets (the "ovector"). You may read the elements in this
       vector, but you must not change any of them.

       For calls to pcre2_match(), the offset_vector field is not (since
       release 10.30) a pointer to the actual ovector that was passed to
       the matching function in the match data block. Instead it points
       to an internal ovector of a size large enough to hold all
       possible captured substrings in the pattern. Note that whenever a
       recursion or subroutine call within a pattern completes, the
       capturing state is reset to what it was before.

       The capture_last field contains the number of the most recently
       captured substring, and the capture_top field contains one more
       than the number of the highest numbered captured substring so
       far. If no substrings have yet been captured, the value of
       capture_last is 0 and the value of capture_top is 1. The values
       of these fields do not always differ by one; for example, when
       the callout in the pattern ((a)(b))(?C2) is taken, capture_last
       is 1 but capture_top is 4.

       The contents of ovector[2] to ovector[<capture_top>*2-1] can be
       inspected in order to extract substrings that have been matched
       so far, in the same way as extracting substrings after a match
       has completed. The values in ovector[0] and ovector[1] are always
       PCRE2_UNSET because the match is by definition not complete.
       Substrings that have not been captured but whose numbers are less
       than capture_top also have both of their ovector slots set to

       For DFA matching, the offset_vector field points to the ovector
       that was passed to the matching function in the match data block
       for callouts at the top level, but to an internal ovector during
       the processing of pattern recursions, lookarounds, and atomic
       groups. However, these ovectors hold no useful information
       because pcre2_dfa_match() does not support substring capturing.
       The value of capture_top is always 1 and the value of
       capture_last is always 0 for DFA matching.

       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.

       The pattern_position field contains the offset in the pattern
       string to the next item to be matched.

       The next_item_length field contains the length of the next item
       to be processed in the pattern string. When the callout is at the
       end of the pattern, the length is zero. When the callout precedes
       an opening parenthesis, the length includes meta characters that
       follow the parenthesis. For example, in a callout before an
       assertion such as (?=ab) the length is 3. For an an alternation
       bar or a closing parenthesis, the length is one, unless a closing
       parenthesis is followed by a quantifier, in which case its length
       is included.  (This changed in release 10.23. In earlier
       releases, before an opening parenthesis the length was that of
       the entire group, and before an alternation bar or a closing
       parenthesis the length was zero.)

       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, and are used by pcre2test to show the next item to
       be matched when displaying callout information.

       In callouts from pcre2_match() the mark field 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 function this field always contains NULL.

       The callout_flags field is always zero in callouts from
       pcre2_dfa_match() or when JIT is being used. When pcre2_match()
       without JIT is used, the following bits may be set:


       This is set for the first callout after the start of matching for
       each new starting position in the subject.


       This is set if there has been a matching backtrack since the
       previous callout, or since the start of matching if this is the
       first callout from a pcre2_match() run.

       Both bits are set when a backtrack has caused a "bumpalong" to a
       new starting position in the subject. Output from pcre2test does
       not indicate the presence of these bits unless the callout_extra
       modifier is set.

       The information in the callout_flags field is provided so that
       applications can track and tell their users how matching with
       backtracking is done. This can be useful when trying to optimize
       patterns, or just to understand how PCRE2 works. There is no
       support in pcre2_dfa_match() because there is no backtracking in
       DFA matching, and there is no support in JIT because JIT is all
       about maximimizing matching performance. In both these cases the
       callout_flags field is always zero.


       The external callout function returns an integer to PCRE2. 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 the matching function returns the
       negative value.

       Negative values should normally be chosen from the set of
       PCRE2_ERROR_xxx values. In particular, PCRE2_ERROR_NOMATCH forces
       a standard "no match" failure. The error number
       PCRE2_ERROR_CALLOUT is reserved for use by callout functions; it
       will never be used by PCRE2 itself.


       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);

       A script language that supports the use of string arguments in
       callouts might like to scan all the callouts in a pattern before
       running the match. This can be done by calling
       pcre2_callout_enumerate(). The first argument is a pointer to a
       compiled pattern, the second points to a callback function, and
       the third is arbitrary user data. The callback function is called
       for every callout in the pattern in the order in which they
       appear. Its first argument is a pointer to a callout enumeration
       block, and its second argument is the user_data value that was
       passed to pcre2_callout_enumerate(). The data block contains the
       following fields:

         version                Block version number
         pattern_position       Offset to next item in pattern
         next_item_length       Length of next item in pattern
         callout_number         Number for numbered callouts
         callout_string_offset  Offset to string within pattern
         callout_string_length  Length of callout string
         callout_string         Points to callout string or is NULL

       The version number is currently 0. It will increase if new fields
       are ever added to the block. The remaining fields are the same as
       their namesakes in the pcre2_callout block that is used for
       callouts during matching, as described above.

       Note that the value of pattern_position is unique for each
       callout.  However, if a callout occurs inside a group that is
       quantified with a non-zero minimum or a fixed maximum, the group
       is replicated inside the compiled pattern. For example, a pattern
       such as /(a){2}/ is compiled as if it were /(a)(a)/. This means
       that the callout will be enumerated more than once, but with the
       same value for pattern_position in each case.

       The callback function should normally return zero. If it returns
       a non-zero value, scanning the pattern stops, and that value is
       returned from pcre2_callout_enumerate().

AUTHOR         top

       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.

REVISION         top

       Last updated: 03 February 2019
       Copyright (c) 1997-2019 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 fetched from
       ⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2023-12-22.  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 man-pages@man7.org

PCRE2 10.33                 03 February 2019             PCRE2CALLOUT(3)

Pages that refer to this page: pcre2grep(1)pcre2test(1)pcre2api(3)pcre2pattern(3)pcre2syntax(3)