NAME
PCRE - Perl-compatible regular expressions
PCRE 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)abc(?C2)def
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
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.
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_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.
THE 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_callout
block. 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 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 pcreapi 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.
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, 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.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 29 September 2009
Copyright (c) 1997-2009 University of Cambridge.