pcre/doc/pcre2callout.3

.TH PCRE2CALLOUT 3 "15 March 2015" "PCRE2 10.20"
.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 *, void *);
.
.SH DESCRIPTION
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.sp
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<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:
.sp
  (?C1)abc(?C"some ""arbitrary"" text")def
.sp
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)
.sp
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)  (?(?C%text%)(?!=d)ab|de)
.sp
This applies only to assertion conditions (because they are themselves
independent groups).
.P
Callouts can be useful for tracking the progress of pattern matching. The
.\" HREF
\fBpcre2test\fP
.\"
program has a pattern qualifier (/auto_callout) that sets automatic callouts.
When any callouts are present, the output from \fBpcre2test\fP 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.
.
.
.SS "Auto-possessification"
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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 compiled
with PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied to the string
"aaaa" is:
.sp
  --->aaaa
   +0 ^        a+
   +2 ^   ^    [bc]
  No match
.sp
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). In this
case, the output changes to this:
.sp
  --->aaaa
   +0 ^        a+
   +2 ^   ^    [bc]
   +2 ^  ^     [bc]
   +2 ^ ^      [bc]
   +2 ^^       [bc]
  No match
.sp
This time, when matching [bc] fails, the matcher backtracks into a+ and tries
again, repeatedly, until a+ itself fails.
.
.
.SS "Automatic .* anchoring"
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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
\fBpcre2_compile()\fP 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.
.P
For example, if the pattern .*\ed is compiled with PCRE2_AUTO_CALLOUT and
applied to the string "aa", the \fBpcre2test\fP output is:
.sp
  --->aa
   +0 ^      .*
   +2 ^ ^    \ed
   +2 ^^     \ed
   +2 ^      \ed
  No match
.sp
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 \fBpcre2_compile()\fP, or starting the
pattern with (*NO_DOTSTAR_ANCHOR). In this case, the output changes to:
.sp
  --->aa
   +0 ^      .*
   +2 ^ ^    \ed
   +2 ^^     \ed
   +2 ^      \ed
   +0  ^     .*
   +2  ^^    \ed
   +2  ^     \ed
  No match
.sp
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.
.P
If a pattern has more than one top-level branch, automatic anchoring occurs if
all branches are anchorable.
.
.
.SS "Other optimizations"
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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, 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
\fBpcre2_callout\fP block. The second argument is the void * callout data that
was supplied when the callout was set up by calling \fBpcre2_set_callout()\fP
(see the
.\" HREF
\fBpcre2api\fP
.\"
documentation). The callout block structure contains the following fields:
.sp
  uint32_t      \fIversion\fP;
  uint32_t      \fIcallout_number\fP;
  uint32_t      \fIcapture_top\fP;
  uint32_t      \fIcapture_last\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;
  PCRE2_SIZE    \fIcallout_string_offset\fP;
  PCRE2_SPTR    \fIcallout_string\fP; 
  uint32_t      \fIcallout_string_length\fP; 
 
.sp
The \fIversion\fP 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.
.
.
.SS "Fields for numerical callouts"
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For a numerical callout, \fIcallout_string\fP is NULL, and \fIcallout_number\fP
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.
.
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.SS "Fields for string callouts"
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For callouts with string arguments, \fIcallout_number\fP is always zero, and
\fIcallout_string\fP points to the string that is contained within the compiled
pattern. Its length is given by \fIcallout_string_length\fP. Duplicated ending
delimiters that were present in the original pattern string have been turned
into single characters. 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 therefore access this delimiter as
\fIcallout_string\fP[-1] if you need it.
.P
The \fIcallout_string_offset\fP 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.
.
.
.SS "Fields for all callouts"
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The remaining fields in the callout block are the same for both kinds of 
callout.
.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 \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, and are used by 
\fBpcre2test\fP to show the next item to be matched when displaying callout 
information.
.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.
.
.
.SH AUTHOR
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.nf
Philip Hazel
University Computing Service
Cambridge, England.
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.
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.SH REVISION
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Last updated: 15 March 2015
Copyright (c) 1997-2015 University of Cambridge.
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