Annotation of embedaddon/pcre/doc/html/pcrecallout.html, revision 1.1
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! 2: <head>
! 3: <title>pcrecallout specification</title>
! 4: </head>
! 5: <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
! 6: <h1>pcrecallout man page</h1>
! 7: <p>
! 8: Return to the <a href="index.html">PCRE index page</a>.
! 9: </p>
! 10: <p>
! 11: This page is part of the PCRE HTML documentation. It was generated automatically
! 12: from the original man page. If there is any nonsense in it, please consult the
! 13: man page, in case the conversion went wrong.
! 14: <br>
! 15: <ul>
! 16: <li><a name="TOC1" href="#SEC1">PCRE CALLOUTS</a>
! 17: <li><a name="TOC2" href="#SEC2">MISSING CALLOUTS</a>
! 18: <li><a name="TOC3" href="#SEC3">THE CALLOUT INTERFACE</a>
! 19: <li><a name="TOC4" href="#SEC4">RETURN VALUES</a>
! 20: <li><a name="TOC5" href="#SEC5">AUTHOR</a>
! 21: <li><a name="TOC6" href="#SEC6">REVISION</a>
! 22: </ul>
! 23: <br><a name="SEC1" href="#TOC1">PCRE CALLOUTS</a><br>
! 24: <P>
! 25: <b>int (*pcre_callout)(pcre_callout_block *);</b>
! 26: </P>
! 27: <P>
! 28: PCRE provides a feature called "callout", which is a means of temporarily
! 29: passing control to the caller of PCRE in the middle of pattern matching. The
! 30: caller of PCRE provides an external function by putting its entry point in the
! 31: global variable <i>pcre_callout</i>. By default, this variable contains NULL,
! 32: which disables all calling out.
! 33: </P>
! 34: <P>
! 35: Within a regular expression, (?C) indicates the points at which the external
! 36: function is to be called. Different callout points can be identified by putting
! 37: a number less than 256 after the letter C. The default value is zero.
! 38: For example, this pattern has two callout points:
! 39: <pre>
! 40: (?C1)abc(?C2)def
! 41: </pre>
! 42: If the PCRE_AUTO_CALLOUT option bit is set when <b>pcre_compile()</b> or
! 43: <b>pcre_compile2()</b> is called, PCRE automatically inserts callouts, all with
! 44: number 255, before each item in the pattern. For example, if PCRE_AUTO_CALLOUT
! 45: is used with the pattern
! 46: <pre>
! 47: A(\d{2}|--)
! 48: </pre>
! 49: it is processed as if it were
! 50: <br>
! 51: <br>
! 52: (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
! 53: <br>
! 54: <br>
! 55: Notice that there is a callout before and after each parenthesis and
! 56: alternation bar. Automatic callouts can be used for tracking the progress of
! 57: pattern matching. The
! 58: <a href="pcretest.html"><b>pcretest</b></a>
! 59: command has an option that sets automatic callouts; when it is used, the output
! 60: indicates how the pattern is matched. This is useful information when you are
! 61: trying to optimize the performance of a particular pattern.
! 62: </P>
! 63: <P>
! 64: The use of callouts in a pattern makes it ineligible for optimization by the
! 65: just-in-time compiler. Studying such a pattern with the PCRE_STUDY_JIT_COMPILE
! 66: option always fails.
! 67: </P>
! 68: <br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br>
! 69: <P>
! 70: You should be aware that, because of optimizations in the way PCRE matches
! 71: patterns by default, callouts sometimes do not happen. For example, if the
! 72: pattern is
! 73: <pre>
! 74: ab(?C4)cd
! 75: </pre>
! 76: PCRE knows that any matching string must contain the letter "d". If the subject
! 77: string is "abyz", the lack of "d" means that matching doesn't ever start, and
! 78: the callout is never reached. However, with "abyd", though the result is still
! 79: no match, the callout is obeyed.
! 80: </P>
! 81: <P>
! 82: If the pattern is studied, PCRE knows the minimum length of a matching string,
! 83: and will immediately give a "no match" return without actually running a match
! 84: if the subject is not long enough, or, for unanchored patterns, if it has
! 85: been scanned far enough.
! 86: </P>
! 87: <P>
! 88: You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
! 89: option to <b>pcre_compile()</b>, <b>pcre_exec()</b>, or <b>pcre_dfa_exec()</b>,
! 90: or by starting the pattern with (*NO_START_OPT). This slows down the matching
! 91: process, but does ensure that callouts such as the example above are obeyed.
! 92: </P>
! 93: <br><a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a><br>
! 94: <P>
! 95: During matching, when PCRE reaches a callout point, the external function
! 96: defined by <i>pcre_callout</i> is called (if it is set). This applies to both
! 97: the <b>pcre_exec()</b> and the <b>pcre_dfa_exec()</b> matching functions. The
! 98: only argument to the callout function is a pointer to a <b>pcre_callout</b>
! 99: block. This structure contains the following fields:
! 100: <pre>
! 101: int <i>version</i>;
! 102: int <i>callout_number</i>;
! 103: int *<i>offset_vector</i>;
! 104: const char *<i>subject</i>;
! 105: int <i>subject_length</i>;
! 106: int <i>start_match</i>;
! 107: int <i>current_position</i>;
! 108: int <i>capture_top</i>;
! 109: int <i>capture_last</i>;
! 110: void *<i>callout_data</i>;
! 111: int <i>pattern_position</i>;
! 112: int <i>next_item_length</i>;
! 113: const unsigned char *<i>mark</i>;
! 114: </pre>
! 115: The <i>version</i> field is an integer containing the version number of the
! 116: block format. The initial version was 0; the current version is 2. The version
! 117: number will change again in future if additional fields are added, but the
! 118: intention is never to remove any of the existing fields.
! 119: </P>
! 120: <P>
! 121: The <i>callout_number</i> field contains the number of the callout, as compiled
! 122: into the pattern (that is, the number after ?C for manual callouts, and 255 for
! 123: automatically generated callouts).
! 124: </P>
! 125: <P>
! 126: The <i>offset_vector</i> field is a pointer to the vector of offsets that was
! 127: passed by the caller to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. When
! 128: <b>pcre_exec()</b> is used, the contents can be inspected in order to extract
! 129: substrings that have been matched so far, in the same way as for extracting
! 130: substrings after a match has completed. For <b>pcre_dfa_exec()</b> this field is
! 131: not useful.
! 132: </P>
! 133: <P>
! 134: The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
! 135: that were passed to <b>pcre_exec()</b>.
! 136: </P>
! 137: <P>
! 138: The <i>start_match</i> field normally contains the offset within the subject at
! 139: which the current match attempt started. However, if the escape sequence \K
! 140: has been encountered, this value is changed to reflect the modified starting
! 141: point. If the pattern is not anchored, the callout function may be called
! 142: several times from the same point in the pattern for different starting points
! 143: in the subject.
! 144: </P>
! 145: <P>
! 146: The <i>current_position</i> field contains the offset within the subject of the
! 147: current match pointer.
! 148: </P>
! 149: <P>
! 150: When the <b>pcre_exec()</b> function is used, the <i>capture_top</i> field
! 151: contains one more than the number of the highest numbered captured substring so
! 152: far. If no substrings have been captured, the value of <i>capture_top</i> is
! 153: one. This is always the case when <b>pcre_dfa_exec()</b> is used, because it
! 154: does not support captured substrings.
! 155: </P>
! 156: <P>
! 157: The <i>capture_last</i> field contains the number of the most recently captured
! 158: substring. If no substrings have been captured, its value is -1. This is always
! 159: the case when <b>pcre_dfa_exec()</b> is used.
! 160: </P>
! 161: <P>
! 162: The <i>callout_data</i> field contains a value that is passed to
! 163: <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> specifically so that it can be
! 164: passed back in callouts. It is passed in the <i>pcre_callout</i> field of the
! 165: <b>pcre_extra</b> data structure. If no such data was passed, the value of
! 166: <i>callout_data</i> in a <b>pcre_callout</b> block is NULL. There is a
! 167: description of the <b>pcre_extra</b> structure in the
! 168: <a href="pcreapi.html"><b>pcreapi</b></a>
! 169: documentation.
! 170: </P>
! 171: <P>
! 172: The <i>pattern_position</i> field is present from version 1 of the
! 173: <i>pcre_callout</i> structure. It contains the offset to the next item to be
! 174: matched in the pattern string.
! 175: </P>
! 176: <P>
! 177: The <i>next_item_length</i> field is present from version 1 of the
! 178: <i>pcre_callout</i> structure. It contains the length of the next item to be
! 179: matched in the pattern string. When the callout immediately precedes an
! 180: alternation bar, a closing parenthesis, or the end of the pattern, the length
! 181: is zero. When the callout precedes an opening parenthesis, the length is that
! 182: of the entire subpattern.
! 183: </P>
! 184: <P>
! 185: The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
! 186: help in distinguishing between different automatic callouts, which all have the
! 187: same callout number. However, they are set for all callouts.
! 188: </P>
! 189: <P>
! 190: The <i>mark</i> field is present from version 2 of the <i>pcre_callout</i>
! 191: structure. In callouts from <b>pcre_exec()</b> it contains a pointer to the
! 192: zero-terminated name of the most recently passed (*MARK), (*PRUNE), or (*THEN)
! 193: item in the match, or NULL if no such items have been passed. Instances of
! 194: (*PRUNE) or (*THEN) without a name do not obliterate a previous (*MARK). In
! 195: callouts from <b>pcre_dfa_exec()</b> this field always contains NULL.
! 196: </P>
! 197: <br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
! 198: <P>
! 199: The external callout function returns an integer to PCRE. If the value is zero,
! 200: matching proceeds as normal. If the value is greater than zero, matching fails
! 201: at the current point, but the testing of other matching possibilities goes
! 202: ahead, just as if a lookahead assertion had failed. If the value is less than
! 203: zero, the match is abandoned, and <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>
! 204: returns the negative value.
! 205: </P>
! 206: <P>
! 207: Negative values should normally be chosen from the set of PCRE_ERROR_xxx
! 208: values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
! 209: The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
! 210: it will never be used by PCRE itself.
! 211: </P>
! 212: <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
! 213: <P>
! 214: Philip Hazel
! 215: <br>
! 216: University Computing Service
! 217: <br>
! 218: Cambridge CB2 3QH, England.
! 219: <br>
! 220: </P>
! 221: <br><a name="SEC6" href="#TOC1">REVISION</a><br>
! 222: <P>
! 223: Last updated: 30 November 2011
! 224: <br>
! 225: Copyright © 1997-2011 University of Cambridge.
! 226: <br>
! 227: <p>
! 228: Return to the <a href="index.html">PCRE index page</a>.
! 229: </p>
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