Annotation of embedaddon/pcre/doc/pcrecallout.3, revision 1.1
1.1 ! misho 1: .TH PCRECALLOUT 3
! 2: .SH NAME
! 3: PCRE - Perl-compatible regular expressions
! 4: .SH "PCRE CALLOUTS"
! 5: .rs
! 6: .sp
! 7: .B int (*pcre_callout)(pcre_callout_block *);
! 8: .PP
! 9: PCRE provides a feature called "callout", which is a means of temporarily
! 10: passing control to the caller of PCRE in the middle of pattern matching. The
! 11: caller of PCRE provides an external function by putting its entry point in the
! 12: global variable \fIpcre_callout\fP. By default, this variable contains NULL,
! 13: which disables all calling out.
! 14: .P
! 15: Within a regular expression, (?C) indicates the points at which the external
! 16: function is to be called. Different callout points can be identified by putting
! 17: a number less than 256 after the letter C. The default value is zero.
! 18: For example, this pattern has two callout points:
! 19: .sp
! 20: (?C1)abc(?C2)def
! 21: .sp
! 22: If the PCRE_AUTO_CALLOUT option bit is set when \fBpcre_compile()\fP or
! 23: \fBpcre_compile2()\fP is called, PCRE automatically inserts callouts, all with
! 24: number 255, before each item in the pattern. For example, if PCRE_AUTO_CALLOUT
! 25: is used with the pattern
! 26: .sp
! 27: A(\ed{2}|--)
! 28: .sp
! 29: it is processed as if it were
! 30: .sp
! 31: (?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
! 32: .sp
! 33: Notice that there is a callout before and after each parenthesis and
! 34: alternation bar. Automatic callouts can be used for tracking the progress of
! 35: pattern matching. The
! 36: .\" HREF
! 37: \fBpcretest\fP
! 38: .\"
! 39: command has an option that sets automatic callouts; when it is used, the output
! 40: indicates how the pattern is matched. This is useful information when you are
! 41: trying to optimize the performance of a particular pattern.
! 42: .P
! 43: The use of callouts in a pattern makes it ineligible for optimization by the
! 44: just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE
! 45: option always fails.
! 46: .
! 47: .
! 48: .SH "MISSING CALLOUTS"
! 49: .rs
! 50: .sp
! 51: You should be aware that, because of optimizations in the way PCRE matches
! 52: patterns by default, callouts sometimes do not happen. For example, if the
! 53: pattern is
! 54: .sp
! 55: ab(?C4)cd
! 56: .sp
! 57: PCRE knows that any matching string must contain the letter "d". If the subject
! 58: string is "abyz", the lack of "d" means that matching doesn't ever start, and
! 59: the callout is never reached. However, with "abyd", though the result is still
! 60: no match, the callout is obeyed.
! 61: .P
! 62: If the pattern is studied, PCRE knows the minimum length of a matching string,
! 63: and will immediately give a "no match" return without actually running a match
! 64: if the subject is not long enough, or, for unanchored patterns, if it has
! 65: been scanned far enough.
! 66: .P
! 67: You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
! 68: option to \fBpcre_compile()\fP, \fBpcre_exec()\fP, or \fBpcre_dfa_exec()\fP,
! 69: or by starting the pattern with (*NO_START_OPT). This slows down the matching
! 70: process, but does ensure that callouts such as the example above are obeyed.
! 71: .
! 72: .
! 73: .SH "THE CALLOUT INTERFACE"
! 74: .rs
! 75: .sp
! 76: During matching, when PCRE reaches a callout point, the external function
! 77: defined by \fIpcre_callout\fP is called (if it is set). This applies to both
! 78: the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The
! 79: only argument to the callout function is a pointer to a \fBpcre_callout\fP
! 80: block. This structure contains the following fields:
! 81: .sp
! 82: int \fIversion\fP;
! 83: int \fIcallout_number\fP;
! 84: int *\fIoffset_vector\fP;
! 85: const char *\fIsubject\fP;
! 86: int \fIsubject_length\fP;
! 87: int \fIstart_match\fP;
! 88: int \fIcurrent_position\fP;
! 89: int \fIcapture_top\fP;
! 90: int \fIcapture_last\fP;
! 91: void *\fIcallout_data\fP;
! 92: int \fIpattern_position\fP;
! 93: int \fInext_item_length\fP;
! 94: const unsigned char *\fImark\fP;
! 95: .sp
! 96: The \fIversion\fP field is an integer containing the version number of the
! 97: block format. The initial version was 0; the current version is 2. The version
! 98: number will change again in future if additional fields are added, but the
! 99: intention is never to remove any of the existing fields.
! 100: .P
! 101: The \fIcallout_number\fP field contains the number of the callout, as compiled
! 102: into the pattern (that is, the number after ?C for manual callouts, and 255 for
! 103: automatically generated callouts).
! 104: .P
! 105: The \fIoffset_vector\fP field is a pointer to the vector of offsets that was
! 106: passed by the caller to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When
! 107: \fBpcre_exec()\fP is used, the contents can be inspected in order to extract
! 108: substrings that have been matched so far, in the same way as for extracting
! 109: substrings after a match has completed. For \fBpcre_dfa_exec()\fP this field is
! 110: not useful.
! 111: .P
! 112: The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values
! 113: that were passed to \fBpcre_exec()\fP.
! 114: .P
! 115: The \fIstart_match\fP field normally contains the offset within the subject at
! 116: which the current match attempt started. However, if the escape sequence \eK
! 117: has been encountered, this value is changed to reflect the modified starting
! 118: point. If the pattern is not anchored, the callout function may be called
! 119: several times from the same point in the pattern for different starting points
! 120: in the subject.
! 121: .P
! 122: The \fIcurrent_position\fP field contains the offset within the subject of the
! 123: current match pointer.
! 124: .P
! 125: When the \fBpcre_exec()\fP function is used, the \fIcapture_top\fP field
! 126: contains one more than the number of the highest numbered captured substring so
! 127: far. If no substrings have been captured, the value of \fIcapture_top\fP is
! 128: one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it
! 129: does not support captured substrings.
! 130: .P
! 131: The \fIcapture_last\fP field contains the number of the most recently captured
! 132: substring. If no substrings have been captured, its value is -1. This is always
! 133: the case when \fBpcre_dfa_exec()\fP is used.
! 134: .P
! 135: The \fIcallout_data\fP field contains a value that is passed to
! 136: \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be
! 137: passed back in callouts. It is passed in the \fIpcre_callout\fP field of the
! 138: \fBpcre_extra\fP data structure. If no such data was passed, the value of
! 139: \fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a
! 140: description of the \fBpcre_extra\fP structure in the
! 141: .\" HREF
! 142: \fBpcreapi\fP
! 143: .\"
! 144: documentation.
! 145: .P
! 146: The \fIpattern_position\fP field is present from version 1 of the
! 147: \fIpcre_callout\fP structure. It contains the offset to the next item to be
! 148: matched in the pattern string.
! 149: .P
! 150: The \fInext_item_length\fP field is present from version 1 of the
! 151: \fIpcre_callout\fP structure. It contains the length of the next item to be
! 152: matched in the pattern string. When the callout immediately precedes an
! 153: alternation bar, a closing parenthesis, or the end of the pattern, the length
! 154: is zero. When the callout precedes an opening parenthesis, the length is that
! 155: of the entire subpattern.
! 156: .P
! 157: The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to
! 158: help in distinguishing between different automatic callouts, which all have the
! 159: same callout number. However, they are set for all callouts.
! 160: .P
! 161: The \fImark\fP field is present from version 2 of the \fIpcre_callout\fP
! 162: structure. In callouts from \fBpcre_exec()\fP it contains a pointer to the
! 163: zero-terminated name of the most recently passed (*MARK), (*PRUNE), or (*THEN)
! 164: item in the match, or NULL if no such items have been passed. Instances of
! 165: (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In
! 166: callouts from \fBpcre_dfa_exec()\fP this field always contains NULL.
! 167: .
! 168: .
! 169: .SH "RETURN VALUES"
! 170: .rs
! 171: .sp
! 172: The external callout function returns an integer to PCRE. If the value is zero,
! 173: matching proceeds as normal. If the value is greater than zero, matching fails
! 174: at the current point, but the testing of other matching possibilities goes
! 175: ahead, just as if a lookahead assertion had failed. If the value is less than
! 176: zero, the match is abandoned, and \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP
! 177: returns the negative value.
! 178: .P
! 179: Negative values should normally be chosen from the set of PCRE_ERROR_xxx
! 180: values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
! 181: The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
! 182: it will never be used by PCRE itself.
! 183: .
! 184: .
! 185: .SH AUTHOR
! 186: .rs
! 187: .sp
! 188: .nf
! 189: Philip Hazel
! 190: University Computing Service
! 191: Cambridge CB2 3QH, England.
! 192: .fi
! 193: .
! 194: .
! 195: .SH REVISION
! 196: .rs
! 197: .sp
! 198: .nf
! 199: Last updated: 30 November 2011
! 200: Copyright (c) 1997-2011 University of Cambridge.
! 201: .fi
FreeBSD-CVSweb <freebsd-cvsweb@FreeBSD.org>
![[BACK]](/icons/cvsweb/back.gif){kind=icon}
Return to pcrecallout.3 CVS log ![[TXT]](/icons/cvsweb/text.gif){kind=icon}
![[DIR]](/icons/cvsweb/dir.gif){kind=icon}
Up to [ELWIX - Embedded LightWeight unIX -] / embedaddon / pcre / doc