PCRECALLOUT(3) Introduction to Library Functions 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
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 (
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. If the pattern contains a conditional group whose
condition is an assertion, an automatic callout is inserted
immediately before the condition. Such a callout may also be inserted
explicitly, for example:
(?(?C9)(?=a)ab|de)
This applies only to assertion conditions (because they are
themselves independent groups).
Automatic callouts can be used for tracking the progress of pattern
matching. The
pcretest program has a pattern qualifier (/C) that
sets automatic callouts; when it is used, the output indicates how
the pattern is being 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
compiles and matches patterns, callouts sometimes do not happen
exactly as you might expect.
At compile time, PCRE "auto-possessifies" repeated items when it
knows that what follows cannot be part of the repeat. For example,
a+[bc] is compiled as if it were a++[bc]. The
pcretest output when
this pattern is anchored and then applied with automatic callouts to
the string "aaaa" is:
--->aaaa
+0 ^ ^
+1 ^ a+
+3 ^ ^ [bc]
No match
This indicates that when matching [bc] fails, there is no
backtracking into a+ and therefore the callouts that would be taken
for the backtracks do not occur. You can disable the auto-possessify
feature by passing PCRE_NO_AUTO_POSSESS to
pcre_compile(), or
starting the pattern with (*NO_AUTO_POSSESS). If this is done in
pcretest (using the /O qualifier), the output changes to this:
--->aaaa
+0 ^ ^
+1 ^ a+
+3 ^ ^ [bc]
+3 ^ ^ [bc]
+3 ^ ^ [bc]
+3 ^^ [bc]
No match
This time, when matching [bc] fails, the matcher backtracks into a+
and tries again, repeatedly, until a+ itself fails.
Other optimizations that provide fast "no match" results also affect
callouts. 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 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
function 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
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 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
captured 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
captured substring. However, when a recursion exits, the value
reverts to what it was outside the recursion, as do the values of all
captured substrings. If no substrings have been captured, the value
of
capture_last 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
functions 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
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: 12 November 2013
Copyright (c) 1997-2013 University of Cambridge.
PCRE 8.34 12 November 2013 PCRECALLOUT(3)
