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    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">SYNOPSIS</a>
   17 <li><a name="TOC2" href="#SEC2">DESCRIPTION</a>
   18 <li><a name="TOC3" href="#SEC3">MISSING CALLOUTS</a>
   19 <li><a name="TOC4" href="#SEC4">THE CALLOUT INTERFACE</a>
   20 <li><a name="TOC5" href="#SEC5">RETURN VALUES</a>
   21 <li><a name="TOC6" href="#SEC6">AUTHOR</a>
   22 <li><a name="TOC7" href="#SEC7">REVISION</a>
   23 </ul>
   24 <br><a name="SEC1" href="#TOC1">SYNOPSIS</a><br>
   25 <P>
   26 <b>#include &#60;pcre.h&#62;</b>
   27 </P>
   28 <P>
   29 <b>int (*pcre_callout)(pcre_callout_block *);</b>
   30 </P>
   31 <P>
   32 <b>int (*pcre16_callout)(pcre16_callout_block *);</b>
   33 </P>
   34 <P>
   35 <b>int (*pcre32_callout)(pcre32_callout_block *);</b>
   36 </P>
   37 <br><a name="SEC2" href="#TOC1">DESCRIPTION</a><br>
   38 <P>
   39 PCRE provides a feature called "callout", which is a means of temporarily
   40 passing control to the caller of PCRE in the middle of pattern matching. The
   41 caller of PCRE provides an external function by putting its entry point in the
   42 global variable <i>pcre_callout</i> (<i>pcre16_callout</i> for the 16-bit
   43 library, <i>pcre32_callout</i> for the 32-bit library). By default, this
   44 variable contains NULL, which disables all calling out.
   45 </P>
   46 <P>
   47 Within a regular expression, (?C) indicates the points at which the external
   48 function is to be called. Different callout points can be identified by putting
   49 a number less than 256 after the letter C. The default value is zero.
   50 For example, this pattern has two callout points:
   51 <pre>
   52   (?C1)abc(?C2)def
   53 </pre>
   54 If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, PCRE
   55 automatically inserts callouts, all with number 255, before each item in the
   56 pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern
   57 <pre>
   58   A(\d{2}|--)
   59 </pre>
   60 it is processed as if it were
   61 <br>
   62 <br>
   63 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
   64 <br>
   65 <br>
   66 Notice that there is a callout before and after each parenthesis and
   67 alternation bar. If the pattern contains a conditional group whose condition is
   68 an assertion, an automatic callout is inserted immediately before the
   69 condition. Such a callout may also be inserted explicitly, for example:
   70 <pre>
   71   (?(?C9)(?=a)ab|de)
   72 </pre>
   73 This applies only to assertion conditions (because they are themselves
   74 independent groups).
   75 </P>
   76 <P>
   77 Automatic callouts can be used for tracking the progress of pattern matching.
   78 The
   79 <a href="pcretest.html"><b>pcretest</b></a>
   80 program has a pattern qualifier (/C) that sets automatic callouts; when it is
   81 used, the output indicates how the pattern is being matched. This is useful
   82 information when you are trying to optimize the performance of a particular
   83 pattern.
   84 </P>
   85 <br><a name="SEC3" href="#TOC1">MISSING CALLOUTS</a><br>
   86 <P>
   87 You should be aware that, because of optimizations in the way PCRE compiles and
   88 matches patterns, callouts sometimes do not happen exactly as you might expect.
   89 </P>
   90 <P>
   91 At compile time, PCRE "auto-possessifies" repeated items when it knows that
   92 what follows cannot be part of the repeat. For example, a+[bc] is compiled as
   93 if it were a++[bc]. The <b>pcretest</b> output when this pattern is anchored and
   94 then applied with automatic callouts to the string "aaaa" is:
   95 <pre>
   96   ---&#62;aaaa
   97    +0 ^        ^
   98    +1 ^        a+
   99    +3 ^   ^    [bc]
  100   No match
  101 </pre>
  102 This indicates that when matching [bc] fails, there is no backtracking into a+
  103 and therefore the callouts that would be taken for the backtracks do not occur.
  104 You can disable the auto-possessify feature by passing PCRE_NO_AUTO_POSSESS
  105 to <b>pcre_compile()</b>, or starting the pattern with (*NO_AUTO_POSSESS). If
  106 this is done in <b>pcretest</b> (using the /O qualifier), the output changes to
  107 this:
  108 <pre>
  109   ---&#62;aaaa
  110    +0 ^        ^
  111    +1 ^        a+
  112    +3 ^   ^    [bc]
  113    +3 ^  ^     [bc]
  114    +3 ^ ^      [bc]
  115    +3 ^^       [bc]
  116   No match
  117 </pre>
  118 This time, when matching [bc] fails, the matcher backtracks into a+ and tries
  119 again, repeatedly, until a+ itself fails.
  120 </P>
  121 <P>
  122 Other optimizations that provide fast "no match" results also affect callouts.
  123 For example, if the pattern is
  124 <pre>
  125   ab(?C4)cd
  126 </pre>
  127 PCRE knows that any matching string must contain the letter "d". If the subject
  128 string is "abyz", the lack of "d" means that matching doesn't ever start, and
  129 the callout is never reached. However, with "abyd", though the result is still
  130 no match, the callout is obeyed.
  131 </P>
  132 <P>
  133 If the pattern is studied, PCRE knows the minimum length of a matching string,
  134 and will immediately give a "no match" return without actually running a match
  135 if the subject is not long enough, or, for unanchored patterns, if it has
  136 been scanned far enough.
  137 </P>
  138 <P>
  139 You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
  140 option to the matching function, or by starting the pattern with
  141 (*NO_START_OPT). This slows down the matching process, but does ensure that
  142 callouts such as the example above are obeyed.
  143 </P>
  144 <br><a name="SEC4" href="#TOC1">THE CALLOUT INTERFACE</a><br>
  145 <P>
  146 During matching, when PCRE reaches a callout point, the external function
  147 defined by <i>pcre_callout</i> or <i>pcre[16|32]_callout</i> is called (if it is
  148 set). This applies to both normal and DFA matching. The only argument to the
  149 callout function is a pointer to a <b>pcre_callout</b> or
  150 <b>pcre[16|32]_callout</b> block. These structures contains the following
  151 fields:
  152 <pre>
  153   int           <i>version</i>;
  154   int           <i>callout_number</i>;
  155   int          *<i>offset_vector</i>;
  156   const char   *<i>subject</i>;           (8-bit version)
  157   PCRE_SPTR16   <i>subject</i>;           (16-bit version)
  158   PCRE_SPTR32   <i>subject</i>;           (32-bit version)
  159   int           <i>subject_length</i>;
  160   int           <i>start_match</i>;
  161   int           <i>current_position</i>;
  162   int           <i>capture_top</i>;
  163   int           <i>capture_last</i>;
  164   void         *<i>callout_data</i>;
  165   int           <i>pattern_position</i>;
  166   int           <i>next_item_length</i>;
  167   const unsigned char *<i>mark</i>;       (8-bit version)
  168   const PCRE_UCHAR16  *<i>mark</i>;       (16-bit version)
  169   const PCRE_UCHAR32  *<i>mark</i>;       (32-bit version)
  170 </pre>
  171 The <i>version</i> field is an integer containing the version number of the
  172 block format. The initial version was 0; the current version is 2. The version
  173 number will change again in future if additional fields are added, but the
  174 intention is never to remove any of the existing fields.
  175 </P>
  176 <P>
  177 The <i>callout_number</i> field contains the number of the callout, as compiled
  178 into the pattern (that is, the number after ?C for manual callouts, and 255 for
  179 automatically generated callouts).
  180 </P>
  181 <P>
  182 The <i>offset_vector</i> field is a pointer to the vector of offsets that was
  183 passed by the caller to the matching function. When <b>pcre_exec()</b> or
  184 <b>pcre[16|32]_exec()</b> is used, the contents can be inspected, in order to
  185 extract substrings that have been matched so far, in the same way as for
  186 extracting substrings after a match has completed. For the DFA matching
  187 functions, this field is not useful.
  188 </P>
  189 <P>
  190 The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
  191 that were passed to the matching function.
  192 </P>
  193 <P>
  194 The <i>start_match</i> field normally contains the offset within the subject at
  195 which the current match attempt started. However, if the escape sequence \K
  196 has been encountered, this value is changed to reflect the modified starting
  197 point. If the pattern is not anchored, the callout function may be called
  198 several times from the same point in the pattern for different starting points
  199 in the subject.
  200 </P>
  201 <P>
  202 The <i>current_position</i> field contains the offset within the subject of the
  203 current match pointer.
  204 </P>
  205 <P>
  206 When the <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b> is used, the
  207 <i>capture_top</i> field contains one more than the number of the highest
  208 numbered captured substring so far. If no substrings have been captured, the
  209 value of <i>capture_top</i> is one. This is always the case when the DFA
  210 functions are used, because they do not support captured substrings.
  211 </P>
  212 <P>
  213 The <i>capture_last</i> field contains the number of the most recently captured
  214 substring. However, when a recursion exits, the value reverts to what it was
  215 outside the recursion, as do the values of all captured substrings. If no
  216 substrings have been captured, the value of <i>capture_last</i> is -1. This is
  217 always the case for the DFA matching functions.
  218 </P>
  219 <P>
  220 The <i>callout_data</i> field contains a value that is passed to a matching
  221 function specifically so that it can be passed back in callouts. It is passed
  222 in the <i>callout_data</i> field of a <b>pcre_extra</b> or <b>pcre[16|32]_extra</b>
  223 data structure. If no such data was passed, the value of <i>callout_data</i> in
  224 a callout block is NULL. There is a description of the <b>pcre_extra</b>
  225 structure in the
  226 <a href="pcreapi.html"><b>pcreapi</b></a>
  227 documentation.
  228 </P>
  229 <P>
  230 The <i>pattern_position</i> field is present from version 1 of the callout
  231 structure. It contains the offset to the next item to be matched in the pattern
  232 string.
  233 </P>
  234 <P>
  235 The <i>next_item_length</i> field is present from version 1 of the callout
  236 structure. It contains the length of the next item to be matched in the pattern
  237 string. When the callout immediately precedes an alternation bar, a closing
  238 parenthesis, or the end of the pattern, the length is zero. When the callout
  239 precedes an opening parenthesis, the length is that of the entire subpattern.
  240 </P>
  241 <P>
  242 The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
  243 help in distinguishing between different automatic callouts, which all have the
  244 same callout number. However, they are set for all callouts.
  245 </P>
  246 <P>
  247 The <i>mark</i> field is present from version 2 of the callout structure. In
  248 callouts from <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b> it contains a
  249 pointer to the zero-terminated name of the most recently passed (*MARK),
  250 (*PRUNE), or (*THEN) item in the match, or NULL if no such items have been
  251 passed. Instances of (*PRUNE) or (*THEN) without a name do not obliterate a
  252 previous (*MARK). In callouts from the DFA matching functions this field always
  253 contains NULL.
  254 </P>
  255 <br><a name="SEC5" href="#TOC1">RETURN VALUES</a><br>
  256 <P>
  257 The external callout function returns an integer to PCRE. If the value is zero,
  258 matching proceeds as normal. If the value is greater than zero, matching fails
  259 at the current point, but the testing of other matching possibilities goes
  260 ahead, just as if a lookahead assertion had failed. If the value is less than
  261 zero, the match is abandoned, the matching function returns the negative value.
  262 </P>
  263 <P>
  264 Negative values should normally be chosen from the set of PCRE_ERROR_xxx
  265 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.
  266 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
  267 it will never be used by PCRE itself.
  268 </P>
  269 <br><a name="SEC6" href="#TOC1">AUTHOR</a><br>
  270 <P>
  271 Philip Hazel
  272 <br>
  273 University Computing Service
  274 <br>
  275 Cambridge CB2 3QH, England.
  276 <br>
  277 </P>
  278 <br><a name="SEC7" href="#TOC1">REVISION</a><br>
  279 <P>
  280 Last updated: 12 November 2013
  281 <br>
  282 Copyright &copy; 1997-2013 University of Cambridge.
  283 <br>
  284 <p>
  285 Return to the <a href="index.html">PCRE index page</a>.
  286 </p>