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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
46  <b>pcre_callout</b> block. This contains the following variables:  <pre>
47  </P>    A(\d{2}|--)
48  <P>  </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    <br><a name="SEC2" href="#TOC1">MISSING CALLOUTS</a><br>
64    <P>
65    You should be aware that, because of optimizations in the way PCRE matches
66    patterns by default, callouts sometimes do not happen. For example, if the
67    pattern is
68    <pre>
69      ab(?C4)cd
70    </pre>
71    PCRE knows that any matching string must contain the letter "d". If the subject
72    string is "abyz", the lack of "d" means that matching doesn't ever start, and
73    the callout is never reached. However, with "abyd", though the result is still
74    no match, the callout is obeyed.
75    </P>
76    <P>
77    If the pattern is studied, PCRE knows the minimum length of a matching string,
78    and will immediately give a "no match" return without actually running a match
79    if the subject is not long enough, or, for unanchored patterns, if it has
80    been scanned far enough.
81    </P>
82    <P>
83    You can disable these optimizations by passing the PCRE_NO_START_OPTIMIZE
84    option to <b>pcre_compile()</b>, <b>pcre_exec()</b>, or <b>pcre_dfa_exec()</b>,
85    or by starting the pattern with (*NO_START_OPT). This slows down the matching
86    process, but does ensure that callouts such as the example above are obeyed.
87    </P>
88    <br><a name="SEC3" href="#TOC1">THE CALLOUT INTERFACE</a><br>
89    <P>
90    During matching, when PCRE reaches a callout point, the external function
91    defined by <i>pcre_callout</i> is called (if it is set). This applies to both
92    the <b>pcre_exec()</b> and the <b>pcre_dfa_exec()</b> matching functions. The
93    only argument to the callout function is a pointer to a <b>pcre_callout</b>
94    block. This structure contains the following fields:
95  <pre>  <pre>
96    int          <i>version</i>;    int          <i>version</i>;
97    int          <i>callout_number</i>;    int          <i>callout_number</i>;
# Line 49  set), the external function is called. I Line 103  set), the external function is called. I
103    int          <i>capture_top</i>;    int          <i>capture_top</i>;
104    int          <i>capture_last</i>;    int          <i>capture_last</i>;
105    void        *<i>callout_data</i>;    void        *<i>callout_data</i>;
106  </PRE>    int          <i>pattern_position</i>;
107  </P>    int          <i>next_item_length</i>;
108  <P>  </pre>
109  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
110  block format. The current version is zero. The version number may change in  block format. The initial version was 0; the current version is 1. The version
111  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
112  of the existing fields.  intention is never to remove any of the existing fields.
113  </P>  </P>
114  <P>  <P>
115  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
116  into the pattern (that is, the number after ?C).  into the pattern (that is, the number after ?C for manual callouts, and 255 for
117    automatically generated callouts).
118  </P>  </P>
119  <P>  <P>
120  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
121  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
122  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
123  for extracting substrings after a match has completed.  substrings that have been matched so far, in the same way as for extracting
124    substrings after a match has completed. For <b>pcre_dfa_exec()</b> this field is
125    not useful.
126  </P>  </P>
127  <P>  <P>
128  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
129  that were passed to <b>pcre_exec()</b>.  that were passed to <b>pcre_exec()</b>.
130  </P>  </P>
131  <P>  <P>
132  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
133  current match attempt started. If the pattern is not anchored, the callout  which the current match attempt started. However, if the escape sequence \K
134  function may be called several times for different starting points.  has been encountered, this value is changed to reflect the modified starting
135    point. If the pattern is not anchored, the callout function may be called
136    several times from the same point in the pattern for different starting points
137    in the subject.
138  </P>  </P>
139  <P>  <P>
140  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
141  current match pointer.  current match pointer.
142  </P>  </P>
143  <P>  <P>
144  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
145  numbered captured substring so far. If no substrings have been captured,  contains one more than the number of the highest numbered captured substring so
146  the value of <i>capture_top</i> is one.  far. If no substrings have been captured, the value of <i>capture_top</i> is
147    one. This is always the case when <b>pcre_dfa_exec()</b> is used, because it
148    does not support captured substrings.
149  </P>  </P>
150  <P>  <P>
151  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
152  substring.  substring. If no substrings have been captured, its value is -1. This is always
153    the case when <b>pcre_dfa_exec()</b> is used.
154  </P>  </P>
155  <P>  <P>
156  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
157  <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
158  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
159  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
160  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
161  <b>pcre_extra</b> structure in the <b>pcreapi</b> documentation.  description of the <b>pcre_extra</b> structure in the
162  </P>  <a href="pcreapi.html"><b>pcreapi</b></a>
163  <br><a name="SEC2" href="#TOC1">RETURN VALUES</a><br>  documentation.
164  <P>  </P>
165  The callout function returns an integer. If the value is zero, matching  <P>
166  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
167  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
168  if a lookahead assertion had failed. If the value is less than zero, the match  matched in the pattern string.
169  is abandoned, and <b>pcre_exec()</b> returns the value.  </P>
170    <P>
171    The <i>next_item_length</i> field is present from version 1 of the
172    <i>pcre_callout</i> structure. It contains the length of the next item to be
173    matched in the pattern string. When the callout immediately precedes an
174    alternation bar, a closing parenthesis, or the end of the pattern, the length
175    is zero. When the callout precedes an opening parenthesis, the length is that
176    of the entire subpattern.
177    </P>
178    <P>
179    The <i>pattern_position</i> and <i>next_item_length</i> fields are intended to
180    help in distinguishing between different automatic callouts, which all have the
181    same callout number. However, they are set for all callouts.
182    </P>
183    <br><a name="SEC4" href="#TOC1">RETURN VALUES</a><br>
184    <P>
185    The external callout function returns an integer to PCRE. If the value is zero,
186    matching proceeds as normal. If the value is greater than zero, matching fails
187    at the current point, but the testing of other matching possibilities goes
188    ahead, just as if a lookahead assertion had failed. If the value is less than
189    zero, the match is abandoned, and <b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>
190    returns the negative value.
191  </P>  </P>
192  <P>  <P>
193  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 195  values. In particular, PCRE_ERROR_NOMATC
195  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;
196  it will never be used by PCRE itself.  it will never be used by PCRE itself.
197  </P>  </P>
198    <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
199    <P>
200    Philip Hazel
201    <br>
202    University Computing Service
203    <br>
204    Cambridge CB2 3QH, England.
205    <br>
206    </P>
207    <br><a name="SEC6" href="#TOC1">REVISION</a><br>
208  <P>  <P>
209  Last updated: 21 January 2003  Last updated: 21 November 2010
210    <br>
211    Copyright &copy; 1997-2010 University of Cambridge.
212  <br>  <br>
213  Copyright &copy; 1997-2003 University of Cambridge.  <p>
214    Return to the <a href="index.html">PCRE index page</a>.
215    </p>

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