PCRE2 — Perl-compatible regular expressions (revised API)
#include <pcre2.h>
int( |
*pcre2_callout)(pcre2_callout_block *, |
void
*) ; |
PCRE2 provides a feature called "callout", which is a
means of temporarily passing control to the caller of PCRE2
in the middle of pattern matching. The caller of PCRE2
provides an external function by putting its entry point in a
match context (see pcre2_set_callout
()) in the pcre2api(3)
documentation).
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 PCRE2_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE2 automatically inserts callouts, all with number 255, before each item in the pattern. For example, if PCRE2_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 pcre2test(3) program has a pattern qualifier (/auto_callout) 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 PCRE2 compiles and matches patterns, callouts sometimes do not happen exactly as you might expect.
At compile time, PCRE2 "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 pcre2test
output
when this pattern is compiled with PCRE2_ANCHORED and
PCRE2_AUTO_CALLOUT and then applied to the string "aaaa"
is:
--->aaaa +0 ^ a+ +2 ^ ^ [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
PCRE2_NO_AUTO_POSSESS to pcre2_compile
(), or starting the pattern
with (*NO_AUTO_POSSESS). In this case, the output changes
to this:
--->aaaa +0 ^ a+ +2 ^ ^ [bc] +2 ^ ^ [bc] +2 ^ ^ [bc] +2 ^^ [bc] No match
This time, when matching [bc] fails, the matcher backtracks into a+ and tries again, repeatedly, until a+ itself fails.
By default, an optimization is applied when .* is the
first significant item in a pattern. If PCRE2_DOTALL is
set, so that the dot can match any character, the pattern
is automatically anchored. If PCRE2_DOTALL is not set, a
match can start only after an internal newline or at the
beginning of the subject, and pcre2_compile
() remembers this. This
optimization is disabled, however, if .* is in an atomic
group or if there is a back reference to the capturing
group in which it appears. It is also disabled if the
pattern contains (*PRUNE) or (*SKIP). However, the presence
of callouts does not affect it.
For example, if the pattern .*\d is compiled with
PCRE2_AUTO_CALLOUT and applied to the string "aa", the
pcre2test
output is:
--->aa +0 ^ .* +2 ^ ^ \d +2 ^^ \d +2 ^ \d No match
This shows that all match attempts start at the
beginning of the subject. In other words, the pattern is
anchored. You can disable this optimization by passing
PCRE2_NO_DOTSTAR_ANCHOR to pcre2_compile
(), or starting the pattern
with (*NO_DOTSTAR_ANCHOR). In this case, the output changes
to:
--->aa +0 ^ .* +2 ^ ^ \d +2 ^^ \d +2 ^ \d +0 ^ .* +2 ^^ \d +2 ^ \d No match
This shows more match attempts, starting at the second subject character. Another optimization, described in the next section, means that there is no subsequent attempt to match with an empty subject.
If a pattern has more than one top-level branch, automatic anchoring occurs if all branches are anchorable.
Other optimizations that provide fast "no match" results also affect callouts. For example, if the pattern is
ab(?C4)cd
PCRE2 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.
PCRE2 also 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
PCRE2_NO_START_OPTIMIZE option to pcre2_compile
(), 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 PCRE2 reaches a callout point, if an
external function is set in the match context, it is called.
This applies to both normal and DFA matching. The first
argument to the callout function is a pointer to a
pcre2_callout
block. The second
argument is the void * callout data that was supplied when
the callout was set up by calling pcre2_set_callout
() (see the pcre2api(3) documentation).
The callout block structure contains the following
fields:
uint32_tversion
; uint32_tcallout_number
; uint32_tcapture_top
; uint32_tcapture_last
; PCRE2_SIZE *offset_vector
; PCRE2_SPTRmark
; PCRE2_SPTRsubject
; PCRE2_SIZEsubject_length
; PCRE2_SIZEstart_match
; PCRE2_SIZEcurrent_position
; PCRE2_SIZEpattern_position
; PCRE2_SIZEnext_item_length
;
The version
field contains
the version number of the block format. The current version
is 0. The version number will change 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 capturing offsets (the "ovector")
that was passed to the matching function in the match data
block. When pcre2_match
() 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 function, 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 pcre2_match
() 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 0. This is
always the case for the DFA matching functions.
The pattern_position
field
contains the offset to the next item to be matched in the
pattern string.
The next_item_length
field
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.
In callouts from pcre2_match
() the mark
field 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 function this field always
contains NULL.
The external callout function returns an integer to PCRE2. 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, and the matching function returns the negative value.
Negative values should normally be chosen from the set of PCRE2_ERROR_xxx values. In particular, PCRE2_ERROR_NOMATCH forces a standard "no match" failure. The error number PCRE2_ERROR_CALLOUT is reserved for use by callout functions; it will never be used by PCRE2 itself.
Last updated: 02 January 2015 Copyright (c) 1997-2015 University of Cambridge.
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This manual page is taken from the PCRE library, which is distributed under the BSD license. |