.TH PCRECALLOUT 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PCRE CALLOUTS"
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.B int (*pcre_callout)(pcre_callout_block *);
.PP
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 \fIpcre_callout\fP. By default, this variable contains NULL,
which disables all calling out.
.P
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:
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  (?C1)abc(?C2)def
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If the PCRE_AUTO_CALLOUT option bit is set when \fBpcre_compile()\fP or
\fBpcre_compile2()\fP 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
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  A(\ed{2}|--)
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it is processed as if it were
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(?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
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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
.\" HREF
\fBpcretest\fP
.\"
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.
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.SH "MISSING CALLOUTS"
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You should be aware that, because of optimizations in the way PCRE matches
patterns by default, callouts sometimes do not happen. For example, if the
pattern is
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  ab(?C4)cd
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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.
.P
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.
.P
You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
option to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. This slows down the
matching process, but does ensure that callouts such as the example above are
obeyed.
.
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.SH "THE CALLOUT INTERFACE"
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During matching, when PCRE reaches a callout point, the external function
defined by \fIpcre_callout\fP is called (if it is set). This applies to both
the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The
only argument to the callout function is a pointer to a \fBpcre_callout\fP
block. This structure contains the following fields:
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  int          \fIversion\fP;
  int          \fIcallout_number\fP;
  int         *\fIoffset_vector\fP;
  const char  *\fIsubject\fP;
  int          \fIsubject_length\fP;
  int          \fIstart_match\fP;
  int          \fIcurrent_position\fP;
  int          \fIcapture_top\fP;
  int          \fIcapture_last\fP;
  void        *\fIcallout_data\fP;
  int          \fIpattern_position\fP;
  int          \fInext_item_length\fP;
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The \fIversion\fP 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.
.P
The \fIcallout_number\fP 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).
.P
The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
passed by the caller to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When
\fBpcre_exec()\fP 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 \fBpcre_dfa_exec()\fP this field is
not useful.
.P
The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
that were passed to \fBpcre_exec()\fP.
.P
The \fIstart_match\fP field normally contains the offset within the subject at
which the current match attempt started. However, if the escape sequence \eK
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.
.P
The \fIcurrent_position\fP field contains the offset within the subject of the
current match pointer.
.P
When the \fBpcre_exec()\fP function is used, the \fIcapture_top\fP field
contains one more than the number of the highest numbered captured substring so
far. If no substrings have been captured, the value of \fIcapture_top\fP is
one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it
does not support captured substrings.
.P
The \fIcapture_last\fP 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 \fBpcre_dfa_exec()\fP is used.
.P
The \fIcallout_data\fP field contains a value that is passed to
\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be
passed back in callouts. It is passed in the \fIpcre_callout\fP field of the
\fBpcre_extra\fP data structure. If no such data was passed, the value of
\fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a
description of the \fBpcre_extra\fP structure in the
.\" HREF
\fBpcreapi\fP
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documentation.
.P
The \fIpattern_position\fP field is present from version 1 of the
\fIpcre_callout\fP structure. It contains the offset to the next item to be
matched in the pattern string.
.P
The \fInext_item_length\fP field is present from version 1 of the
\fIpcre_callout\fP 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.
.P
The \fIpattern_position\fP and \fInext_item_length\fP 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.
.
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.SH "RETURN VALUES"
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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 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP
returns the negative value.
.P
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.
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.SH AUTHOR
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Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
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.SH REVISION
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Last updated: 29 September 2009
Copyright (c) 1997-2009 University of Cambridge.
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