Add the pcre2callout documentation.
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Philip.Hazel
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.TH PCRE2CALLOUT 3 "19 October 2014" "PCRE2 10.00"
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.SH NAME
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PCRE2 - Perl-compatible regular expressions (revised API)
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.SH SYNOPSIS
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.rs
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.sp
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.B #include <pcre2.h>
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.PP
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.SM
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.B int (*pcre2_callout)(pcre2_callout_block *);
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.
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.SH DESCRIPTION
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.rs
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.sp
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PCRE2 provides a feature called "callout", which is a means of temporarily
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passing control to the caller of PCRE2 in the middle of pattern matching. The
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caller of PCRE2 provides an external function by putting its entry point in
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a match context (see \fBpcre2_set_callout()\fP) in the
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.\" HREF
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\fBpcre2api\fP
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.\"
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documentation).
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.P
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Within a regular expression, (?C) indicates the points at which the external
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function is to be called. Different callout points can be identified by putting
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a number less than 256 after the letter C. The default value is zero.
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For example, this pattern has two callout points:
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.sp
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(?C1)abc(?C2)def
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.sp
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If the PCRE2_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE2
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automatically inserts callouts, all with number 255, before each item in the
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pattern. For example, if PCRE2_AUTO_CALLOUT is used with the pattern
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.sp
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A(\ed{2}|--)
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.sp
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it is processed as if it were
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.sp
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(?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
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.sp
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Notice that there is a callout before and after each parenthesis and
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alternation bar. If the pattern contains a conditional group whose condition is
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an assertion, an automatic callout is inserted immediately before the
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condition. Such a callout may also be inserted explicitly, for example:
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.sp
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(?(?C9)(?=a)ab|de)
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.sp
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This applies only to assertion conditions (because they are themselves
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independent groups).
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.P
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Automatic callouts can be used for tracking the progress of pattern matching.
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The
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.\" HREF
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\fBpcre2test\fP
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.\"
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program has a pattern qualifier (/auto_callout) that sets automatic callouts;
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when it is used, the output indicates how the pattern is being matched. This is
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useful information when you are trying to optimize the performance of a
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particular pattern.
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.
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.
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.SH "MISSING CALLOUTS"
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.rs
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.sp
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You should be aware that, because of optimizations in the way PCRE2 compiles
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and matches patterns, callouts sometimes do not happen exactly as you might
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expect.
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.P
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At compile time, PCRE2 "auto-possessifies" repeated items when it knows that
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what follows cannot be part of the repeat. For example, a+[bc] is compiled as
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if it were a++[bc]. The \fBpcre2test\fP output when this pattern is anchored
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and then applied with automatic callouts to the string "aaaa" is:
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.sp
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--->aaaa
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+0 ^ ^
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+1 ^ a+
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+3 ^ ^ [bc]
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No match
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.sp
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This indicates that when matching [bc] fails, there is no backtracking into a+
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and therefore the callouts that would be taken for the backtracks do not occur.
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You can disable the auto-possessify feature by passing PCRE2_NO_AUTO_POSSESS
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to \fBpcre2_compile()\fP, or starting the pattern with (*NO_AUTO_POSSESS). If
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this is done in \fBpcre2test\fP (using the /no_auto_possess qualifier), the
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output changes to this:
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.sp
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--->aaaa
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+0 ^ ^
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+1 ^ a+
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+3 ^ ^ [bc]
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+3 ^ ^ [bc]
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+3 ^ ^ [bc]
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+3 ^^ [bc]
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No match
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.sp
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This time, when matching [bc] fails, the matcher backtracks into a+ and tries
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again, repeatedly, until a+ itself fails.
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.P
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Other optimizations that provide fast "no match" results also affect callouts.
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For example, if the pattern is
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.sp
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ab(?C4)cd
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.sp
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PCRE2 knows that any matching string must contain the letter "d". If the
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subject string is "abyz", the lack of "d" means that matching doesn't ever
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start, and the callout is never reached. However, with "abyd", though the
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result is still no match, the callout is obeyed.
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.P
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PCRE2 also knows the minimum length of a matching string, and will immediately
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give a "no match" return without actually running a match if the subject is not
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long enough, or, for unanchored patterns, if it has been scanned far enough.
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.P
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You can disable these optimizations by passing the PCRE2_NO_START_OPTIMIZE
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option to the matching function, or by starting the pattern with
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(*NO_START_OPT). This slows down the matching process, but does ensure that
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callouts such as the example above are obeyed.
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.
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.
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.SH "THE CALLOUT INTERFACE"
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.rs
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.sp
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During matching, when PCRE2 reaches a callout point, the external function that
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is set in the match context is called (if it is set). This applies to both
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normal and DFA matching. The only argument to the callout function is a pointer
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to a \fBpcre2_callout\fP block. This structure contains the following fields:
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.sp
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uint32_t \fIversion\fP;
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uint32_t \fIcallout_number\fP;
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uint32_t \fIcapture_top\fP;
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uint32_t \fIcapture_last\fP;
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void *\fIcallout_data\fP;
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PCRE2_SIZE *\fIoffset_vector\fP;
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PCRE2_SPTR \fImark\fP;
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PCRE2_SPTR \fIsubject\fP;
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PCRE2_SIZE \fIsubject_length\fP;
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PCRE2_SIZE \fIstart_match\fP;
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PCRE2_SIZE \fIcurrent_position\fP;
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PCRE2_SIZE \fIpattern_position\fP;
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PCRE2_SIZE \fInext_item_length\fP;
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.sp
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The \fIversion\fP field contains the version number of the block format. The
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current version is 0. The version number will change in future if additional
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fields are added, but the intention is never to remove any of the existing
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fields.
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.P
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The \fIcallout_number\fP field contains the number of the callout, as compiled
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into the pattern (that is, the number after ?C for manual callouts, and 255 for
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automatically generated callouts).
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.P
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The \fIoffset_vector\fP field is a pointer to the vector of capturing offsets
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(the "ovector") that was passed to the matching function in the match data
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block. When \fBpcre2_match()\fP is used, the contents can be inspected, in
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order to extract substrings that have been matched so far, in the same way as
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for extracting substrings after a match has completed. For the DFA matching
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function, this field is not useful.
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.P
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The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
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that were passed to the matching function.
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.P
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The \fIstart_match\fP field normally contains the offset within the subject at
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which the current match attempt started. However, if the escape sequence \eK
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has been encountered, this value is changed to reflect the modified starting
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point. If the pattern is not anchored, the callout function may be called
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several times from the same point in the pattern for different starting points
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in the subject.
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.P
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The \fIcurrent_position\fP field contains the offset within the subject of the
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current match pointer.
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.P
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When the \fBpcre2_match()\fP is used, the \fIcapture_top\fP field contains one
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more than the number of the highest numbered captured substring so far. If no
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substrings have been captured, the value of \fIcapture_top\fP is one. This is
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always the case when the DFA functions are used, because they do not support
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captured substrings.
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.P
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The \fIcapture_last\fP field contains the number of the most recently captured
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substring. However, when a recursion exits, the value reverts to what it was
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outside the recursion, as do the values of all captured substrings. If no
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substrings have been captured, the value of \fIcapture_last\fP is 0. This is
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always the case for the DFA matching functions.
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.P
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The \fIcallout_data\fP field contains a value that is passed to a matching
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function specifically so that it can be passed back in callouts. It is set in
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the match context when the callout is set up by calling
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\fBpcre2_set_callout()\fP (see the
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.\" HREF
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\fBpcre2api\fP
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.\"
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documentation).
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.P
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The \fIpattern_position\fP field contains the offset to the next item to be
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matched in the pattern string.
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.P
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The \fInext_item_length\fP field contains the length of the next item to be
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matched in the pattern string. When the callout immediately precedes an
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alternation bar, a closing parenthesis, or the end of the pattern, the length
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is zero. When the callout precedes an opening parenthesis, the length is that
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of the entire subpattern.
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.P
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The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
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help in distinguishing between different automatic callouts, which all have the
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same callout number. However, they are set for all callouts.
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.P
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In callouts from \fBpcre2_match()\fP the \fImark\fP field contains a pointer to
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the zero-terminated name of the most recently passed (*MARK), (*PRUNE), or
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(*THEN) item in the match, or NULL if no such items have been passed. Instances
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of (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In
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callouts from the DFA matching function this field always contains NULL.
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.
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.
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.SH "RETURN VALUES"
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.rs
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.sp
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The external callout function returns an integer to PCRE2. If the value is
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zero, matching proceeds as normal. If the value is greater than zero, matching
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fails at the current point, but the testing of other matching possibilities
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goes ahead, just as if a lookahead assertion had failed. If the value is less
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than zero, the match is abandoned, and the matching function returns the
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negative value.
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.P
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Negative values should normally be chosen from the set of PCRE2_ERROR_xxx
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values. In particular, PCRE2_ERROR_NOMATCH forces a standard "no match"
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failure. The error number PCRE2_ERROR_CALLOUT is reserved for use by callout
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functions; it will never be used by PCRE2 itself.
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.
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.
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.SH AUTHOR
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.rs
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.sp
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.nf
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Philip Hazel
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University Computing Service
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Cambridge CB2 3QH, England.
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.fi
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.
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.
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.SH REVISION
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.rs
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.sp
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.nf
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Last updated: 19 October 2014
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Copyright (c) 1997-2014 University of Cambridge.
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.fi
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