244 lines
9.4 KiB
Groff
244 lines
9.4 KiB
Groff
.TH PCRE2CALLOUT 3 "19 October 2014" "PCRE2 10.00"
|
|
.SH NAME
|
|
PCRE2 - Perl-compatible regular expressions (revised API)
|
|
.SH SYNOPSIS
|
|
.rs
|
|
.sp
|
|
.B #include <pcre2.h>
|
|
.PP
|
|
.SM
|
|
.B int (*pcre2_callout)(pcre2_callout_block *);
|
|
.
|
|
.SH DESCRIPTION
|
|
.rs
|
|
.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) 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
|
|
.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)
|
|
.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"
|
|
.rs
|
|
.sp
|
|
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
|
|
.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). 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
|
|
.sp
|
|
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
|
|
.sp
|
|
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 the matching function, 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"
|
|
.rs
|
|
.sp
|
|
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:
|
|
.sp
|
|
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"
|
|
.rs
|
|
.sp
|
|
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
|
|
.rs
|
|
.sp
|
|
.nf
|
|
Philip Hazel
|
|
University Computing Service
|
|
Cambridge CB2 3QH, England.
|
|
.fi
|
|
.
|
|
.
|
|
.SH REVISION
|
|
.rs
|
|
.sp
|
|
.nf
|
|
Last updated: 19 October 2014
|
|
Copyright (c) 1997-2014 University of Cambridge.
|
|
.fi
|