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.