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 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
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
it is processed as if it were
Notice that there is a callout before and after each parenthesis and
alternation bar. If the pattern contains a conditional group whose con-
dition is an assertion, an automatic callout is inserted immediately
before the condition. Such a callout may also be inserted explicitly,
This applies only to assertion conditions (because they are themselves
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 pat-
tern is being matched. This is useful information when you are trying
to optimize the performance of a particular pattern.
+0 ^ ^
+1 ^ a+
+3 ^ ^ [bc]
This indicates that when matching [bc] fails, there is no backtracking
into a+ and therefore the callouts that would be taken for the back-
tracks 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:
+0 ^ ^
+1 ^ a+
+3 ^ ^ [bc]
+3 ^ ^ [bc]
+3 ^ ^ [bc]
+3 ^^ [bc]
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
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
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. 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 cap-
ture_last is -1. This is always the case for the DFA matching func-
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 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.
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
University Computing Service
Cambridge CB2 3QH, England.
Last updated: 12 November 2013
Copyright (c) 1997-2013 University of Cambridge.
PCRE 8.34 12 November 2013 PCRECALLOUT(3)
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