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diff --git a/doc/pcrecallout.3 b/doc/pcrecallout.3 new file mode 100644 index 0000000..8ebc995 --- /dev/null +++ b/doc/pcrecallout.3 @@ -0,0 +1,255 @@ +.TH PCRECALLOUT 3 "12 November 2013" "PCRE 8.34" +.SH NAME +PCRE - Perl-compatible regular expressions +.SH SYNOPSIS +.rs +.sp +.B #include <pcre.h> +.PP +.SM +.B int (*pcre_callout)(pcre_callout_block *); +.PP +.B int (*pcre16_callout)(pcre16_callout_block *); +.PP +.B int (*pcre32_callout)(pcre32_callout_block *); +. +.SH DESCRIPTION +.rs +.sp +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 (\fIpcre16_callout\fP for the 16-bit +library, \fIpcre32_callout\fP for the 32-bit library). 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 a pattern is compiled, 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. 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 +\fBpcretest\fP +.\" +program has a pattern qualifier (/C) 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 PCRE compiles and +matches patterns, callouts sometimes do not happen exactly as you might expect. +.P +At compile time, PCRE "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 \fBpcretest\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 PCRE_NO_AUTO_POSSESS +to \fBpcre_compile()\fP, or starting the pattern with (*NO_AUTO_POSSESS). If +this is done in \fBpcretest\fP (using the /O 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 +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 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 PCRE reaches a callout point, the external function +defined by \fIpcre_callout\fP or \fIpcre[16|32]_callout\fP 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 \fBpcre_callout\fP or +\fBpcre[16|32]_callout\fP block. These structures contains the following +fields: +.sp + int \fIversion\fP; + int \fIcallout_number\fP; + int *\fIoffset_vector\fP; + const char *\fIsubject\fP; (8-bit version) + PCRE_SPTR16 \fIsubject\fP; (16-bit version) + PCRE_SPTR32 \fIsubject\fP; (32-bit version) + 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; (8-bit version) + const PCRE_UCHAR16 *\fImark\fP; (16-bit version) + const PCRE_UCHAR32 *\fImark\fP; (32-bit version) +.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 the matching function. When \fBpcre_exec()\fP or +\fBpcre[16|32]_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 the DFA matching +functions, 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 \fBpcre_exec()\fP or \fBpcre[16|32]_exec()\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 -1. 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 passed +in the \fIcallout_data\fP field of a \fBpcre_extra\fP or \fBpcre[16|32]_extra\fP +data structure. If no such data was passed, the value of \fIcallout_data\fP in +a callout 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 callout +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 callout +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 callout structure. In +callouts from \fBpcre_exec()\fP or \fBpcre[16|32]_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 the DFA matching functions 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, the matching function 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: 12 November 2013 +Copyright (c) 1997-2013 University of Cambridge. +.fi |