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NAME | SYNOPSIS | DESCRIPTION | MISSING CALLOUTS | THE CALLOUT INTERFACE | RETURN VALUES | AUTHOR | REVISION | COLOPHON |
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PCRECALLOUT(3) Library Functions Manual PCRECALLOUT(3)
PCRE - Perl-compatible regular expressions
#include <pcre.h>
int (*pcre_callout)(pcre_callout_block *);
int (*pcre16_callout)(pcre16_callout_block *);
int (*pcre32_callout)(pcre32_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 (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.
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.
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.
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.
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
Last updated: 12 November 2013
Copyright (c) 1997-2013 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular
Expressions) project. Information about the project can be found
at ⟨http://www.pcre.org/⟩. If you have a bug report for this
manual page, see
⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩. This page was
obtained from the tarball pcre-8.45.tar.gz fetched from
⟨ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/⟩ on
2021-08-27. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
PCRE 8.34 12 November 2013 PCRECALLOUT(3)
Pages that refer to this page: pcretest(1), pcreapi(3), pcrepattern(3), pcresyntax(3)
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