pcrecallout(3)
PCRECALLOUT(3) C 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 cal-
lout 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 condi-
tion. Such a callout may also be inserted explicitly, for
example:
(?(?C9)(?=a)ab|de)
This applies only to assertion conditions (because they are
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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 partic-
ular 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 out-
put 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
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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 run-
ning 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 cal-
louts). 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
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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 dif-
ferent 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 cal-
lout 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 sub-
pattern. 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
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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 func-
tion 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 Last change: 12 November 2013 5
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