PCRECALLOUT(3) Library Functions Manual PCRECALLOUT(3)
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 temporar-
ily 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. Automatic callouts can be used for tracking the
progress of pattern matching. The pcretest 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.
The use of callouts in a pattern makes it ineligible for optimization
by the just-in-time compiler. Studying such a pattern with the
PCRE_STUDY_JIT_COMPILE option always fails.
MISSING CALLOUTS
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
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_OPTI-
MIZE 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 func-
tion 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 com-
piled into the pattern (that is, the number after ?C for manual call-
outs, 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 cap-
tured 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 cap-
tured substring. If no substrings have been captured, its value 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 func-
tions 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 stan-
dard "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: 24 June 2012
Copyright (c) 1997-2012 University of Cambridge.
PCRE 8.30 24 June 2012 PCRECALLOUT(3)
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