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NAME | SYNOPSIS | DESCRIPTION | MISSING CALLOUTS | THE CALLOUT INTERFACE | RETURN VALUES FROM CALLOUTS | CALLOUT ENUMERATION | AUTHOR | REVISION | COLOPHON |
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PCRE2CALLOUT(3) Library Functions Manual PCRE2CALLOUT(3)
PCRE2 - Perl-compatible regular expressions (revised API)
#include <pcre2.h>
int (*pcre2_callout)(pcre2_callout_block *, void *);
int pcre2_callout_enumerate(const pcre2_code *code,
int (*callback)(pcre2_callout_enumerate_block *, void *),
void *user_data);
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 documentation).
When using the pcre2_substitute() function, an additional callout
feature is available. This does a callout after each change to the
subject string and is described in the pcre2api documentation; the
rest of this document is concerned with callouts during pattern
matching.
Within a regular expression, (?C<arg>) indicates a point 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. Alternatively, the argument
may be a delimited string. The starting delimiter must be one of `
' " ^ % # $ { and the ending delimiter is the same as the start,
except for {, where the ending delimiter is }. If the ending
delimiter is needed within the string, it must be doubled. For
example, this pattern has two callout points:
(?C1)abc(?C"some ""arbitrary"" text")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 except for immediately before
or after an explicit callout. For example, if PCRE2_AUTO_CALLOUT
is used with the pattern
A(?C3)B
it is processed as if it were
(?C255)A(?C3)B(?C255)
Here is a more complicated example:
A(\d{2}|--)
With PCRE2_AUTO_CALLOUT, this pattern 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) (?(?C%text%)(?!=d)ab|de)
This applies only to assertion conditions (because they are
themselves independent groups).
Callouts can be useful for tracking the progress of pattern
matching. The pcre2test program has a pattern qualifier
(/auto_callout) that sets automatic callouts. When any callouts
are present, the output from pcre2test 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.
Auto-possessification
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+ (because it is being treated as 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.
Automatic .* anchoring
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. If a pattern has more than one
top-level branch, automatic anchoring occurs if all branches are
anchorable.
This optimization is disabled, however, if .* is in an atomic
group or if there is a backreference to the capture 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.
Other optimizations
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.
For most patterns 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 provided in the match context, it is called.
This applies to both normal, DFA, and JIT 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 documentation). The callout
block structure contains the following fields, not necessarily in
this order:
uint32_t version;
uint32_t callout_number;
uint32_t capture_top;
uint32_t capture_last;
uint32_t callout_flags;
PCRE2_SIZE *offset_vector;
PCRE2_SPTR mark;
PCRE2_SPTR subject;
PCRE2_SIZE subject_length;
PCRE2_SIZE start_match;
PCRE2_SIZE current_position;
PCRE2_SIZE pattern_position;
PCRE2_SIZE next_item_length;
PCRE2_SIZE callout_string_offset;
PCRE2_SIZE callout_string_length;
PCRE2_SPTR callout_string;
The version field contains the version number of the block format.
The current version is 2; the three callout string fields were
added for version 1, and the callout_flags field for version 2. If
you are writing an application that might use an earlier release
of PCRE2, you should check the version number before accessing any
of these fields. The version number will increase in future if
more fields are added, but the intention is never to remove any of
the existing fields.
Fields for numerical callouts
For a numerical callout, callout_string is NULL, and
callout_number contains the number of the callout, in the range
0-255. This is the number that follows (?C for callouts that part
of the pattern; it is 255 for automatically generated callouts.
Fields for string callouts
For callouts with string arguments, callout_number is always zero,
and callout_string points to the string that is contained within
the compiled pattern. Its length is given by
callout_string_length. Duplicated ending delimiters that were
present in the original pattern string have been turned into
single characters, but there is no other processing of the callout
string argument. An additional code unit containing binary zero is
present after the string, but is not included in the length. The
delimiter that was used to start the string is also stored within
the pattern, immediately before the string itself. You can access
this delimiter as callout_string[-1] if you need it.
The callout_string_offset field is the code unit offset to the
start of the callout argument string within the original pattern
string. This is provided for the benefit of applications such as
script languages that might need to report errors in the callout
string within the pattern.
Fields for all callouts
The remaining fields in the callout block are the same for both
kinds of callout.
The offset_vector field is a pointer to a vector of capturing
offsets (the "ovector"). You may read the elements in this vector,
but you must not change any of them.
For calls to pcre2_match(), the offset_vector field is not (since
release 10.30) a pointer to the actual ovector that was passed to
the matching function in the match data block. Instead it points
to an internal ovector of a size large enough to hold all possible
captured substrings in the pattern. Note that whenever a recursion
or subroutine call within a pattern completes, the capturing state
is reset to what it was before.
The capture_last field contains the number of the most recently
captured substring, and the capture_top field contains one more
than the number of the highest numbered captured substring so far.
If no substrings have yet been captured, the value of capture_last
is 0 and the value of capture_top is 1. The values of these fields
do not always differ by one; for example, when the callout in the
pattern ((a)(b))(?C2) is taken, capture_last is 1 but capture_top
is 4.
The contents of ovector[2] to ovector[<capture_top>*2-1] can be
inspected in order to extract substrings that have been matched so
far, in the same way as extracting substrings after a match has
completed. The values in ovector[0] and ovector[1] are always
PCRE2_UNSET because the match is by definition not complete.
Substrings that have not been captured but whose numbers are less
than capture_top also have both of their ovector slots set to
PCRE2_UNSET.
For DFA matching, the offset_vector field points to the ovector
that was passed to the matching function in the match data block
for callouts at the top level, but to an internal ovector during
the processing of pattern recursions, lookarounds, and atomic
groups. However, these ovectors hold no useful information because
pcre2_dfa_match() does not support substring capturing. The value
of capture_top is always 1 and the value of capture_last is always
0 for DFA matching.
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.
The pattern_position field contains the offset in the pattern
string to the next item to be matched.
The next_item_length field contains the length of the next item to
be processed in the pattern string. When the callout is at the end
of the pattern, the length is zero. When the callout precedes an
opening parenthesis, the length includes meta characters that
follow the parenthesis. For example, in a callout before an
assertion such as (?=ab) the length is 3. For an alternation bar
or a closing parenthesis, the length is one, unless a closing
parenthesis is followed by a quantifier, in which case its length
is included. (This changed in release 10.23. In earlier releases,
before an opening parenthesis the length was that of the entire
group, and before an alternation bar or a closing parenthesis the
length was zero.)
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, and are used by pcre2test to show the next item to be
matched when displaying callout information.
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 callout_flags field is always zero in callouts from
pcre2_dfa_match() or when JIT is being used. When pcre2_match()
without JIT is used, the following bits may be set:
PCRE2_CALLOUT_STARTMATCH
This is set for the first callout after the start of matching for
each new starting position in the subject.
PCRE2_CALLOUT_BACKTRACK
This is set if there has been a matching backtrack since the
previous callout, or since the start of matching if this is the
first callout from a pcre2_match() run.
Both bits are set when a backtrack has caused a "bumpalong" to a
new starting position in the subject. Output from pcre2test does
not indicate the presence of these bits unless the callout_extra
modifier is set.
The information in the callout_flags field is provided so that
applications can track and tell their users how matching with
backtracking is done. This can be useful when trying to optimize
patterns, or just to understand how PCRE2 works. There is no
support in pcre2_dfa_match() because there is no backtracking in
DFA matching, and there is no support in JIT because JIT is all
about maximimizing matching performance. In both these cases the
callout_flags field is always zero.
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.
int pcre2_callout_enumerate(const pcre2_code *code,
int (*callback)(pcre2_callout_enumerate_block *, void *),
void *user_data);
A script language that supports the use of string arguments in
callouts might like to scan all the callouts in a pattern before
running the match. This can be done by calling
pcre2_callout_enumerate(). The first argument is a pointer to a
compiled pattern, the second points to a callback function, and
the third is arbitrary user data. The callback function is called
for every callout in the pattern in the order in which they
appear. Its first argument is a pointer to a callout enumeration
block, and its second argument is the user_data value that was
passed to pcre2_callout_enumerate(). The data block contains the
following fields:
version Block version number
pattern_position Offset to next item in pattern
next_item_length Length of next item in pattern
callout_number Number for numbered callouts
callout_string_offset Offset to string within pattern
callout_string_length Length of callout string
callout_string Points to callout string or is NULL
The version number is currently 0. It will increase if new fields
are ever added to the block. The remaining fields are the same as
their namesakes in the pcre2_callout block that is used for
callouts during matching, as described above.
Note that the value of pattern_position is unique for each
callout. However, if a callout occurs inside a group that is
quantified with a non-zero minimum or a fixed maximum, the group
is replicated inside the compiled pattern. For example, a pattern
such as /(a){2}/ is compiled as if it were /(a)(a)/. This means
that the callout will be enumerated more than once, but with the
same value for pattern_position in each case.
The callback function should normally return zero. If it returns a
non-zero value, scanning the pattern stops, and that value is
returned from pcre2_callout_enumerate().
Philip Hazel
Retired from University Computing Service
Cambridge, England.
Last updated: 19 January 2024
Copyright (c) 1997-2024 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 fetched from
⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2025-08-11. 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
PCRE2 10.46-DEV 19 January 2024 PCRE2CALLOUT(3)
Pages that refer to this page: pcre2grep(1), pcre2test(1), pcre2api(3), pcre2pattern(3), pcre2syntax(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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