pcre/doc/pcre2callout.3

.TH PCRE2CALLOUT 3 "19 October 2014" "PCRE2 10.00"
.SH NAME
PCRE2 - Perl-compatible regular expressions (revised API)
.SH SYNOPSIS
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.B #include <pcre2.h>
.PP
.SM
.B int (*pcre2_callout)(pcre2_callout_block *);
.
.SH DESCRIPTION
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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 \fBpcre2_set_callout()\fP) in the
.\" HREF                                                                  
\fBpcre2api\fP                                           
.\"                                                               
documentation).
.P
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:
.sp
  (?C1)abc(?C2)def
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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
.sp
  A(\ed{2}|--)
.sp
it is processed as if it were
.sp
(?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
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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:
.sp
  (?(?C9)(?=a)ab|de)
.sp
This applies only to assertion conditions (because they are themselves
independent groups).
.P
Automatic callouts can be used for tracking the progress of pattern matching.
The
.\" HREF
\fBpcre2test\fP
.\"
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.
.
.
.SH "MISSING CALLOUTS"
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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.
.P
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 \fBpcre2test\fP output when this pattern is anchored
and then applied with automatic callouts to the string "aaaa" is:
.sp
  --->aaaa
   +0 ^        ^
   +1 ^        a+
   +3 ^   ^    [bc]
  No match
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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 \fBpcre2_compile()\fP, or starting the pattern with (*NO_AUTO_POSSESS). If
this is done in \fBpcre2test\fP (using the /no_auto_possess qualifier), the
output changes to this:
.sp
  --->aaaa
   +0 ^        ^
   +1 ^        a+
   +3 ^   ^    [bc]
   +3 ^  ^     [bc]
   +3 ^ ^      [bc]
   +3 ^^       [bc]
  No match
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This time, when matching [bc] fails, the matcher backtracks into a+ and tries
again, repeatedly, until a+ itself fails.
.P
Other optimizations that provide fast "no match" results also affect callouts.
For example, if the pattern is
.sp
  ab(?C4)cd
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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.
.P
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.
.P
You can disable these optimizations by passing the PCRE2_NO_START_OPTIMIZE
option to \fBpcre2_compile()\fP, 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.
.
.
.SH "THE CALLOUT INTERFACE"
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During matching, when PCRE2 reaches a callout point, the external function that
is set in the match context is called (if it is set). This applies to both
normal and DFA matching. The only argument to the callout function is a pointer
to a \fBpcre2_callout\fP block. This structure contains the following fields:
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  uint32_t      \fIversion\fP;
  uint32_t      \fIcallout_number\fP;
  uint32_t      \fIcapture_top\fP;
  uint32_t      \fIcapture_last\fP;
  void         *\fIcallout_data\fP;
  PCRE2_SIZE   *\fIoffset_vector\fP;
  PCRE2_SPTR    \fImark\fP;
  PCRE2_SPTR    \fIsubject\fP;
  PCRE2_SIZE    \fIsubject_length\fP;
  PCRE2_SIZE    \fIstart_match\fP;
  PCRE2_SIZE    \fIcurrent_position\fP;
  PCRE2_SIZE    \fIpattern_position\fP;
  PCRE2_SIZE    \fInext_item_length\fP;
.sp
The \fIversion\fP 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.
.P
The \fIcallout_number\fP 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).
.P
The \fIoffset_vector\fP 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 \fBpcre2_match()\fP 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.
.P
The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
that were passed to the matching function.
.P
The \fIstart_match\fP field normally contains the offset within the subject at
which the current match attempt started. However, if the escape sequence \eK
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.
.P
The \fIcurrent_position\fP field contains the offset within the subject of the
current match pointer.
.P
When the \fBpcre2_match()\fP is used, the \fIcapture_top\fP field contains one
more than the number of the highest numbered captured substring so far. If no
substrings have been captured, the value of \fIcapture_top\fP is one. This is
always the case when the DFA functions are used, because they do not support
captured substrings.
.P
The \fIcapture_last\fP 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 \fIcapture_last\fP is 0. This is
always the case for the DFA matching functions.
.P
The \fIcallout_data\fP field contains a value that is passed to a matching
function specifically so that it can be passed back in callouts. It is set in
the match context when the callout is set up by calling
\fBpcre2_set_callout()\fP (see the
.\" HREF
\fBpcre2api\fP
.\"
documentation).
.P
The \fIpattern_position\fP field contains the offset to the next item to be
matched in the pattern string.
.P
The \fInext_item_length\fP 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.
.P
The \fIpattern_position\fP and \fInext_item_length\fP 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.
.P
In callouts from \fBpcre2_match()\fP the \fImark\fP 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.
.
.
.SH "RETURN VALUES"
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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.
.P
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.
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.SH AUTHOR
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Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
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.SH REVISION
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Last updated: 19 October 2014
Copyright (c) 1997-2014 University of Cambridge.
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