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<h1>pcrecallout man page</h1>

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<ul>
<li><a name="TOC1" href="#SEC1">PCRE CALLOUTS</a>
<li><a name="TOC2" href="#SEC2">MISSING CALLOUTS</a>
<li><a name="TOC3" href="#SEC3">THE CALLOUT INTERFACE</a>
<li><a name="TOC4" href="#SEC4">RETURN VALUES</a>
<li><a name="TOC5" href="#SEC5">AUTHOR</a>
<li><a name="TOC6" href="#SEC6">REVISION</a>
</ul>
 <a name="SEC1" href="#TOC1">PCRE CALLOUTS</a> 

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:
<pre>
 (?C1)abc(?C2)def
</pre>
If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() or
pcre_compile2() 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
<pre>
 A(\d{2}|--)
</pre>
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
<a href="pcretest.html">pcretest</a>
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.

 <a name="SEC2" href="#TOC1">MISSING CALLOUTS</a> 

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
<pre>
 ab(?C4)cd
</pre>
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 pcre_exec() or pcre_dfa_exec(). This slows down the
matching process, but does ensure that callouts such as the example above are
obeyed.

 <a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a> 

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_callout
block. This structure contains the following fields:
<pre>
 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;
</pre>
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 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() 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
<a href="pcreapi.html">pcreapi</a>
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.

 <a name="SEC4" href="#TOC1">RETURN VALUES</a> 

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.

 <a name="SEC5" href="#TOC1">AUTHOR</a> 

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

 <a name="SEC6" href="#TOC1">REVISION</a> 

Last updated: 29 September 2009
 
Copyright &copy; 1997-2009 University of Cambridge.
 

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