PCRE2CALLOUT(3)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 tempo‐
rarily 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).
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 end‐
ing 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. 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 con‐
dition 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 par‐
ticular 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+ and therefore the callouts that would be taken for the back‐
tracks do not occur. You can disable the auto-possessify feature by
passing PCRE2_NO_AUTO_POSSESS to pcre2_compile(), or starting the pat‐
tern 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. 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). How‐
ever, 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 sub‐
ject. 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 out‐
put 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 charac‐
ter. 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
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_OPTI‐
MIZE 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 set in the match context, it is called. This applies to
both normal and DFA matching. The first argument to the callout func‐
tion 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 call‐
out block structure contains the following fields:
uint32_t version;
uint32_t callout_number;
uint32_t capture_top;
uint32_t capture_last;
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 1; the three callout string fields were added for
this version. If you are writing an application that might use an ear‐
lier 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 manual callouts; it is 255 for automati‐
cally 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 com‐
piled 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 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 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 0. This is always the case for the DFA matching functions.
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
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, and
are used by pcre2test to show the next item to be matched when display‐
ing 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.
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 enumer‐
ation block, and its second argument is the user_data value that was
passed to pcre2_callout_enumerate(). The data block contains the fol‐
lowing 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
University Computing Service
Cambridge, England.
REVISION
Last updated: 23 March 2015
Copyright (c) 1997-2015 University of Cambridge.
PCRE2 10.20 23 March 2015 PCRE2CALLOUT(3)
