Provided by: libpcre2-dev_10.34-7ubuntu0.1_amd64 bug


       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 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

       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 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  pattern  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). 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 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_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 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  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

       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().


       Philip Hazel
       University Computing Service
       Cambridge, England.


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