Tristan Matthews | 0461646 | 2013-11-14 16:09:34 -0500 | [diff] [blame^] | 1 | <html> |
| 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> |