pcrecallout man page
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#SEC1PCRE CALLOUTS #SEC2MISSING CALLOUTS #SEC3THE CALLOUT INTERFACE #SEC4RETURN VALUES #TOC1PCRE CALLOUTS int (*pcre_callout)(pcre_callout_block *);
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. 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)\deabc(?C2)def
If the PCRE_AUTO_CALLOUT option bit is set when 
pcre_compile() is called,
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. Automatic callouts can be used for tracking the progress of
pattern matching. The
pcretest.htmlpcretest command has an option that sets automatic callouts; when it is used, the output
indicates how the pattern is matched. This is useful information when you are
trying to optimize the performance of a particular pattern.
#TOC1MISSING CALLOUTS You should be aware that, because of optimizations in the way PCRE matches
patterns, callouts sometimes do not happen. 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.
#TOC1THE CALLOUT INTERFACE During matching, when PCRE reaches a callout point, the external function
defined by 
pcre_callout is called (if it is set). This applies to both
the 
pcre_exec() and the pcre_dfa_exec() matching functions. The
only argument to the callout function is a pointer to a 
pcre_calloutblock. This structure contains the following fields:
  int          
version;
  int          
callout_number;
  int         *
offset_vector;
  const char  *
subject;
  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;
The 
version field is an integer containing the version number of the
block format. The initial version was 0; the current version is 1. 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 
pcre_exec() or pcre_dfa_exec(). When
pcre_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 
pcre_dfa_exec() this field is
not useful.
The 
subject and subject_length fields contain copies of the values
that were passed to 
pcre_exec().
The 
start_match field contains the offset within the subject at which the
current match attempt started. 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() function 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 
pcre_dfa_exec() is used, because it
does not support captured substrings.
The 
capture_last field contains the number of the most recently captured
substring. If no substrings have been captured, its value is -1. This is always
the case when 
pcre_dfa_exec() is used.
The 
callout_data field contains a value that is passed to
pcre_exec() or pcre_dfa_exec() specifically so that it can be
passed back in callouts. It is passed in the 
pcre_callout field of the
pcre_extra data structure. If no such data was passed, the value of
callout_data in a pcre_callout block is NULL. There is a
description of the 
pcre_extra structure in the
pcreapi.htmlpcreapi documentation.
The 
pattern_position field is present from version 1 of the
pcre_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
pcre_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.
#TOC1RETURN 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, and 
pcre_exec() (or pcre_dfa_exec())
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.
Last updated: 28 February 2005
Copyright © 1997-2005 University of Cambridge.
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