Provided by: libpcre2-dev_10.21-1_amd64 
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).
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. 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) (?(?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+ 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. 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
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.
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 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:
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
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 manual callouts; 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 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 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 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.
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
University Computing Service
Cambridge, England.
REVISION
Last updated: 23 March 2015
Copyright (c) 1997-2015 University of Cambridge.