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+.TH PCRECALLOUT 3
+.SH NAME
+PCRE - Perl-compatible regular expressions
+.SH "PCRE CALLOUTS"
+.rs
+.sp
+.B int (*pcre_callout)(pcre_callout_block *);
+.PP
+PCRE provides a feature called "callout", which is a means of temporarily
+passing control to the caller of PCRE in the middle of pattern matching. The
+caller of PCRE provides an external function by putting its entry point in the
+global variable \fIpcre_callout\fP. By default, this variable contains NULL,
+which disables all calling out.
+.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 PCRE_AUTO_CALLOUT option bit is set when \fBpcre_compile()\fP or
+\fBpcre_compile2()\fP is called, PCRE automatically inserts callouts, all with
+number 255, before each item in the pattern. For example, if PCRE_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. Automatic callouts can be used for tracking the progress of
+pattern matching. The
+.\" HREF
+\fBpcretest\fP
+.\"
+command has an option that sets automatic callouts; when it is used, the output
+indicates how the pattern is matched. This is useful information when you are
+trying to optimize the performance of a particular pattern.
+.P
+The use of callouts in a pattern makes it ineligible for optimization by the
+just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE
+option always fails.
+.
+.
+.SH "MISSING CALLOUTS"
+.rs
+.sp
+You should be aware that, because of optimizations in the way PCRE matches
+patterns by default, callouts sometimes do not happen. For example, if the
+pattern is
+.sp
+  ab(?C4)cd
+.sp
+PCRE 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
+If the pattern is studied, PCRE 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 PCRE_NO_START_OPTIMIZE
+option to \fBpcre_compile()\fP, \fBpcre_exec()\fP, or \fBpcre_dfa_exec()\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"
+.rs
+.sp
+During matching, when PCRE reaches a callout point, the external function
+defined by \fIpcre_callout\fP is called (if it is set). This applies to both
+the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The
+only argument to the callout function is a pointer to a \fBpcre_callout\fP
+block. This structure contains the following fields:
+.sp
+  int         \fIversion\fP;
+  int         \fIcallout_number\fP;
+  int        *\fIoffset_vector\fP;
+  const char *\fIsubject\fP;
+  int         \fIsubject_length\fP;
+  int         \fIstart_match\fP;
+  int         \fIcurrent_position\fP;
+  int         \fIcapture_top\fP;
+  int         \fIcapture_last\fP;
+  void       *\fIcallout_data\fP;
+  int         \fIpattern_position\fP;
+  int         \fInext_item_length\fP;
+  const unsigned char *\fImark\fP;
+.sp
+The \fIversion\fP field is an integer containing the version number of the
+block format. The initial version was 0; the current version is 2. The version
+number will change again 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 offsets that was
+passed by the caller to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When
+\fBpcre_exec()\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 \fBpcre_dfa_exec()\fP this field is
+not useful.
+.P
+The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
+that were passed to \fBpcre_exec()\fP.
+.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 \fBpcre_exec()\fP function 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 \fBpcre_dfa_exec()\fP is used, because it
+does not support captured substrings.
+.P
+The \fIcapture_last\fP field contains the number of the most recently captured
+substring. If no substrings have been captured, its value is -1. This is always
+the case when \fBpcre_dfa_exec()\fP is used.
+.P
+The \fIcallout_data\fP field contains a value that is passed to
+\fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be
+passed back in callouts. It is passed in the \fIpcre_callout\fP field of the
+\fBpcre_extra\fP data structure. If no such data was passed, the value of
+\fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a
+description of the \fBpcre_extra\fP structure in the
+.\" HREF
+\fBpcreapi\fP
+.\"
+documentation.
+.P
+The \fIpattern_position\fP field is present from version 1 of the
+\fIpcre_callout\fP structure. It contains the offset to the next item to be
+matched in the pattern string.
+.P
+The \fInext_item_length\fP field is present from version 1 of the
+\fIpcre_callout\fP structure. It 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
+The \fImark\fP field is present from version 2 of the \fIpcre_callout\fP
+structure. In callouts from \fBpcre_exec()\fP it 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 \fBpcre_dfa_exec()\fP this field always contains NULL.
+.
+.
+.SH "RETURN VALUES"
+.rs
+.sp
+The external callout function returns an integer to PCRE. 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 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP
+returns the negative value.
+.P
+Negative values should normally be chosen from the set of PCRE_ERROR_xxx
+values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
+The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
+it will never be used by PCRE itself.
+.
+.
+.SH AUTHOR
+.rs
+.sp
+.nf
+Philip Hazel
+University Computing Service
+Cambridge CB2 3QH, England.
+.fi
+.
+.
+.SH REVISION
+.rs
+.sp
+.nf
+Last updated: 30 November 2011
+Copyright (c) 1997-2011 University of Cambridge.
+.fi