PCRE2CALLOUT(3) Introduction to Library Functions PCRE2CALLOUT(3)

NAME

PCRE2 - Perl-compatible regular expressions (revised API)

SYNOPSIS

#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);

DESCRIPTION

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.

MISSING CALLOUTS

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.

THE CALLOUT INTERFACE

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.

RETURN VALUES FROM CALLOUTS

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.

CALLOUT ENUMERATION

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().

AUTHOR

Philip Hazel
Retired from University Computing Service
Cambridge, England.

REVISION

Last updated: 19 January 2024
Copyright (c) 1997-2024 University of Cambridge.

PCRE2 10.45 19 January 2024 PCRE2CALLOUT(3)