PCRE2CALLOUT(3)            Library Functions Manual            PCRE2CALLOUT(3)

       PCRE2 - Perl-compatible regular expressions (revised API)


       #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);


       PCRE2  provides  a feature called "callout", which is a means of tempo-
       rarily 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 fea-
       ture 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 end-
       ing delimiter is }. If  the  ending  delimiter  is  needed  within  the
       string,  it  must be doubled. For example, this pattern has two callout

         (?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 con-
       dition  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  par-
       ticular 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 this:

          +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 pat-
       tern has more than one top-level branch, automatic anchoring occurs  if
       all branches are anchorable.

       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). How-
       ever, 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 sub-
       ject. In other words, the pattern is anchored. You can disable this op-
       timization  by  passing  PCRE2_NO_DOTSTAR_ANCHOR to pcre2_compile(), or
       starting the pattern with (*NO_DOTSTAR_ANCHOR). In this case, the  out-
       put 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 charac-
       ter.  Another optimization, described in the next section,  means  that
       there is no subsequent attempt to match with an empty subject.

   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 enough.

       You can disable these optimizations by passing the PCRE2_NO_START_OPTI-
       MIZE  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 call-
       out 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  writ-
       ing  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  com-
       piled 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  re-
       lease  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  in-
       ternal  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  cap-
       tured  substring,  and the capture_top field contains one more than the
       number of the highest numbered captured substring so far.  If  no  sub-
       strings  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  in-
       spected  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 PCRE2_UNSET.

       For DFA matching, the offset_vector field points to  the  ovector  that
       was  passed  to the matching function in the match data block for call-
       outs 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  se-
       quence  \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 paren-
       thesis. 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  quan-
       tifier, 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 clos-
       ing 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 display-
       ing 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 indi-
       cate the presence of these bits unless the  callout_extra  modifier  is

       The information in the callout_flags field is provided so that applica-
       tions 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 sup-
       port 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_ER-
       ROR_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 enumer-
       ation block, and its second argument is the user_data  value  that  was
       passed  to  pcre2_callout_enumerate(). The data block contains the fol-
       lowing 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

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


       Philip Hazel
       University Computing Service
       Cambridge, England.


       Last updated: 03 February 2019
       Copyright (c) 1997-2019 University of Cambridge.

PCRE2 10.33                    03 February 2019                PCRE2CALLOUT(3)
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