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3  <title>pcrecallout specification</title>  <title>pcrecallout specification</title>
4  </head>  </head>
5  <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">  <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6  This HTML document has been generated automatically from the original man page.  <h1>pcrecallout man page</h1>
7  If there is any nonsense in it, please consult the man page, in case the  <p>
8  conversion went wrong.<br>  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>  <ul>
16  <li><a name="TOC1" href="#SEC1">PCRE CALLOUTS</a>  <li><a name="TOC1" href="#SEC1">PCRE CALLOUTS</a>
17  <li><a name="TOC2" href="#SEC2">RETURN VALUES</a>  <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>  </ul>
23  <br><a name="SEC1" href="#TOC1">PCRE CALLOUTS</a><br>  <br><a name="SEC1" href="#TOC1">PCRE CALLOUTS</a><br>
24  <P>  <P>
# Line 26  Within a regular expression, (?C) indica Line 36  Within a regular expression, (?C) indica
36  function is to be called. Different callout points can be identified by putting  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.  a number less than 256 after the letter C. The default value is zero.
38  For example, this pattern has two callout points:  For example, this pattern has two callout points:
 </P>  
 <P>  
39  <pre>  <pre>
40    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
41  </PRE>  </pre>
42  </P>  If the PCRE_AUTO_CALLOUT option bit is set when <b>pcre_compile()</b> or
43  <P>  <b>pcre_compile2()</b> is called, PCRE automatically inserts callouts, all with
44  During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is  number 255, before each item in the pattern. For example, if PCRE_AUTO_CALLOUT
45  set), the external function is called. Its only argument is a pointer to a  is used with the pattern
 <b>pcre_callout</b> block. This contains the following variables:  
 </P>  
 <P>  
46  <pre>  <pre>
47    int          <i>version</i>;    A(\d{2}|--)
48    int          <i>callout_number</i>;  </pre>
49    int         *<i>offset_vector</i>;  it is processed as if it were
50    const char  *<i>subject</i>;  <br>
51    int          <i>subject_length</i>;  <br>
52    int          <i>start_match</i>;  (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
53    int          <i>current_position</i>;  <br>
54    int          <i>capture_top</i>;  <br>
55    int          <i>capture_last</i>;  Notice that there is a callout before and after each parenthesis and
56    void        *<i>callout_data</i>;  alternation bar. Automatic callouts can be used for tracking the progress of
57  </PRE>  pattern matching. The
58  </P>  <a href="pcretest.html"><b>pcretest</b></a>
59  <P>  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  The <i>version</i> field is an integer containing the version number of the
116  block format. The current version is zero. The version number may change in  block format. The initial version was 0; the current version is 2. The version
117  future if additional fields are added, but the intention is never to remove any  number will change again in future if additional fields are added, but the
118  of the existing fields.  intention is never to remove any of the existing fields.
119  </P>  </P>
120  <P>  <P>
121  The <i>callout_number</i> field contains the number of the callout, as compiled  The <i>callout_number</i> field contains the number of the callout, as compiled
122  into the pattern (that is, the number after ?C).  into the pattern (that is, the number after ?C for manual callouts, and 255 for
123    automatically generated callouts).
124  </P>  </P>
125  <P>  <P>
126  The <i>offset_vector</i> field is a pointer to the vector of offsets that was  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>. The contents can be inspected in  passed by the caller to <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. When
128  order to extract substrings that have been matched so far, in the same way as  <b>pcre_exec()</b> is used, the contents can be inspected in order to extract
129  for extracting substrings after a match has completed.  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>  </P>
133  <P>  <P>
134  The <i>subject</i> and <i>subject_length</i> fields contain copies the values  The <i>subject</i> and <i>subject_length</i> fields contain copies of the values
135  that were passed to <b>pcre_exec()</b>.  that were passed to <b>pcre_exec()</b>.
136  </P>  </P>
137  <P>  <P>
138  The <i>start_match</i> field contains the offset within the subject at which the  The <i>start_match</i> field normally contains the offset within the subject at
139  current match attempt started. If the pattern is not anchored, the callout  which the current match attempt started. However, if the escape sequence \K
140  function may be called several times for different starting points.  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>  </P>
145  <P>  <P>
146  The <i>current_position</i> field contains the offset within the subject of the  The <i>current_position</i> field contains the offset within the subject of the
147  current match pointer.  current match pointer.
148  </P>  </P>
149  <P>  <P>
150  The <i>capture_top</i> field contains one more than the number of the highest  When the <b>pcre_exec()</b> function is used, the <i>capture_top</i> field
151  numbered captured substring so far. If no substrings have been captured,  contains one more than the number of the highest numbered captured substring so
152  the value of <i>capture_top</i> is one.  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>  </P>
156  <P>  <P>
157  The <i>capture_last</i> field contains the number of the most recently captured  The <i>capture_last</i> field contains the number of the most recently captured
158  substring.  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>  </P>
161  <P>  <P>
162  The <i>callout_data</i> field contains a value that is passed to  The <i>callout_data</i> field contains a value that is passed to
163  <b>pcre_exec()</b> by the caller specifically so that it can be passed back in  <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b> specifically so that it can be
164  callouts. It is passed in the <i>pcre_callout</i> field of the <b>pcre_extra</b>  passed back in callouts. It is passed in the <i>pcre_callout</i> field of the
165  data structure. If no such data was passed, the value of <i>callout_data</i> in  <b>pcre_extra</b> data structure. If no such data was passed, the value of
166  a <b>pcre_callout</b> block is NULL. There is a description of the  <i>callout_data</i> in a <b>pcre_callout</b> block is NULL. There is a
167  <b>pcre_extra</b> structure in the <b>pcreapi</b> documentation.  description of the <b>pcre_extra</b> structure in the
168  </P>  <a href="pcreapi.html"><b>pcreapi</b></a>
169  <br><a name="SEC2" href="#TOC1">RETURN VALUES</a><br>  documentation.
170  <P>  </P>
171  The callout function returns an integer. If the value is zero, matching  <P>
172  proceeds as normal. If the value is greater than zero, matching fails at the  The <i>pattern_position</i> field is present from version 1 of the
173  current point, but backtracking to test other possibilities goes ahead, just as  <i>pcre_callout</i> structure. It contains the offset to the next item to be
174  if a lookahead assertion had failed. If the value is less than zero, the match  matched in the pattern string.
175  is abandoned, and <b>pcre_exec()</b> returns the value.  </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>  </P>
206  <P>  <P>
207  Negative values should normally be chosen from the set of PCRE_ERROR_xxx  Negative values should normally be chosen from the set of PCRE_ERROR_xxx
# Line 111  values. In particular, PCRE_ERROR_NOMATC Line 209  values. In particular, PCRE_ERROR_NOMATC
209  The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;  The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;
210  it will never be used by PCRE itself.  it will never be used by PCRE itself.
211  </P>  </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>  <P>
223  Last updated: 21 January 2003  Last updated: 30 November 2011
224    <br>
225    Copyright &copy; 1997-2011 University of Cambridge.
226  <br>  <br>
227  Copyright &copy; 1997-2003 University of Cambridge.  <p>
228    Return to the <a href="index.html">PCRE index page</a>.
229    </p>
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