NAME

       PCRE - Perl-compatible regular expressions

SYNOPSIS

       #include <pcre.h>

       int (*pcre_callout)(pcre_callout_block *);

       int (*pcre16_callout)(pcre16_callout_block *);

       int (*pcre32_callout)(pcre32_callout_block *);

DESCRIPTION

       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:

         (?C1)abc(?C2)def

       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

         A(\d{2}|--)

       it is processed as if it were

       (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

       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)

       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.

MISSING CALLOUTS

       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:

         --->aaaa
          +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:

         --->aaaa
          +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

         ab(?C4)cd

       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 enough.

       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.

THE CALLOUT INTERFACE

       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 documentation.

       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

       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

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

REVISION

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