Provided by: libpcre3-dev_8.39-12_amd64 bug

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.