1 .TH PCRECALLOUT 3

     2 .SH NAME

     3 PCRE - Perl-compatible regular expressions

     4 .SH "PCRE CALLOUTS"

     5 .rs

     6 .sp

     7 .B int (*pcre_callout)(pcre_callout_block *);

     8 .PP

     9 PCRE provides a feature called "callout", which is a means of temporarily

    10 passing control to the caller of PCRE in the middle of pattern matching. The

    11 caller of PCRE provides an external function by putting its entry point in the

    12 global variable \fIpcre_callout\fP. By default, this variable contains NULL,

    13 which disables all calling out.

    14 .P

    15 Within a regular expression, (?C) indicates the points at which the external

    16 function is to be called. Different callout points can be identified by putting

    17 a number less than 256 after the letter C. The default value is zero.

    18 For example, this pattern has two callout points:

    19 .sp

    20   (?C1)abc(?C2)def

    21 .sp

    22 If the PCRE_AUTO_CALLOUT option bit is set when \fBpcre_compile()\fP is called,

    23 PCRE automatically inserts callouts, all with number 255, before each item in

    24 the pattern. For example, if PCRE_AUTO_CALLOUT is used with the pattern

    25 .sp

    26   A(\ed{2}|--)

    27 .sp

    28 it is processed as if it were

    29 .sp

    30 (?C255)A(?C255)((?C255)\ed{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

    31 .sp

    32 Notice that there is a callout before and after each parenthesis and

    33 alternation bar. Automatic callouts can be used for tracking the progress of

    34 pattern matching. The

    35 .\" HREF

    36 \fBpcretest\fP

    37 .\"

    38 command has an option that sets automatic callouts; when it is used, the output

    39 indicates how the pattern is matched. This is useful information when you are

    40 trying to optimize the performance of a particular pattern.

    41 .

    42 .

    43 .SH "MISSING CALLOUTS"

    44 .rs

    45 .sp

    46 You should be aware that, because of optimizations in the way PCRE matches

    47 patterns, callouts sometimes do not happen. For example, if the pattern is

    48 .sp

    49   ab(?C4)cd

    50 .sp

    51 PCRE knows that any matching string must contain the letter "d". If the subject

    52 string is "abyz", the lack of "d" means that matching doesn't ever start, and

    53 the callout is never reached. However, with "abyd", though the result is still

    54 no match, the callout is obeyed.

    55 .

    56 .

    57 .SH "THE CALLOUT INTERFACE"

    58 .rs

    59 .sp

    60 During matching, when PCRE reaches a callout point, the external function

    61 defined by \fIpcre_callout\fP is called (if it is set). This applies to both

    62 the \fBpcre_exec()\fP and the \fBpcre_dfa_exec()\fP matching functions. The

    63 only argument to the callout function is a pointer to a \fBpcre_callout\fP

    64 block. This structure contains the following fields:

    65 .sp

    66   int          \fIversion\fP;

    67   int          \fIcallout_number\fP;

    68   int         *\fIoffset_vector\fP;

    69   const char  *\fIsubject\fP;

    70   int          \fIsubject_length\fP;

    71   int          \fIstart_match\fP;

    72   int          \fIcurrent_position\fP;

    73   int          \fIcapture_top\fP;

    74   int          \fIcapture_last\fP;

    75   void        *\fIcallout_data\fP;

    76   int          \fIpattern_position\fP;

    77   int          \fInext_item_length\fP;

    78 .sp

    79 The \fIversion\fP field is an integer containing the version number of the

    80 block format. The initial version was 0; the current version is 1. The version

    81 number will change again in future if additional fields are added, but the

    82 intention is never to remove any of the existing fields.

    83 .P

    84 The \fIcallout_number\fP field contains the number of the callout, as compiled

    85 into the pattern (that is, the number after ?C for manual callouts, and 255 for

    86 automatically generated callouts).

    87 .P

    88 The \fIoffset_vector\fP field is a pointer to the vector of offsets that was

    89 passed by the caller to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. When

    90 \fBpcre_exec()\fP is used, the contents can be inspected in order to extract

    91 substrings that have been matched so far, in the same way as for extracting

    92 substrings after a match has completed. For \fBpcre_dfa_exec()\fP this field is

    93 not useful.

    94 .P

    95 The \fIsubject\fP and \fIsubject_length\fP fields contain copies of the values

    96 that were passed to \fBpcre_exec()\fP.

    97 .P

    98 The \fIstart_match\fP field normally contains the offset within the subject at

    99 which the current match attempt started. However, if the escape sequence \eK

   100 has been encountered, this value is changed to reflect the modified starting

   101 point. If the pattern is not anchored, the callout function may be called

   102 several times from the same point in the pattern for different starting points

   103 in the subject.

   104 .P

   105 The \fIcurrent_position\fP field contains the offset within the subject of the

   106 current match pointer.

   107 .P

   108 When the \fBpcre_exec()\fP function is used, the \fIcapture_top\fP field

   109 contains one more than the number of the highest numbered captured substring so

   110 far. If no substrings have been captured, the value of \fIcapture_top\fP is

   111 one. This is always the case when \fBpcre_dfa_exec()\fP is used, because it

   112 does not support captured substrings.

   113 .P

   114 The \fIcapture_last\fP field contains the number of the most recently captured

   115 substring. If no substrings have been captured, its value is -1. This is always

   116 the case when \fBpcre_dfa_exec()\fP is used.

   117 .P

   118 The \fIcallout_data\fP field contains a value that is passed to

   119 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP specifically so that it can be

   120 passed back in callouts. It is passed in the \fIpcre_callout\fP field of the

   121 \fBpcre_extra\fP data structure. If no such data was passed, the value of

   122 \fIcallout_data\fP in a \fBpcre_callout\fP block is NULL. There is a

   123 description of the \fBpcre_extra\fP structure in the

   124 .\" HREF

   125 \fBpcreapi\fP

   126 .\"

   127 documentation.

   128 .P

   129 The \fIpattern_position\fP field is present from version 1 of the

   130 \fIpcre_callout\fP structure. It contains the offset to the next item to be

   131 matched in the pattern string.

   132 .P

   133 The \fInext_item_length\fP field is present from version 1 of the

   134 \fIpcre_callout\fP structure. It contains the length of the next item to be

   135 matched in the pattern string. When the callout immediately precedes an

   136 alternation bar, a closing parenthesis, or the end of the pattern, the length

   137 is zero. When the callout precedes an opening parenthesis, the length is that

   138 of the entire subpattern.

   139 .P

   140 The \fIpattern_position\fP and \fInext_item_length\fP fields are intended to

   141 help in distinguishing between different automatic callouts, which all have the

   142 same callout number. However, they are set for all callouts.

   143 .

   144 .

   145 .SH "RETURN VALUES"

   146 .rs

   147 .sp

   148 The external callout function returns an integer to PCRE. If the value is zero,

   149 matching proceeds as normal. If the value is greater than zero, matching fails

   150 at the current point, but the testing of other matching possibilities goes

   151 ahead, just as if a lookahead assertion had failed. If the value is less than

   152 zero, the match is abandoned, and \fBpcre_exec()\fP (or \fBpcre_dfa_exec()\fP)

   153 returns the negative value.

   154 .P

   155 Negative values should normally be chosen from the set of PCRE_ERROR_xxx

   156 values. In particular, PCRE_ERROR_NOMATCH forces a standard "no match" failure.

   157 The error number PCRE_ERROR_CALLOUT is reserved for use by callout functions;

   158 it will never be used by PCRE itself.

   159 .

   160 .

   161 .SH AUTHOR

   162 .rs

   163 .sp

   164 .nf

   165 Philip Hazel

   166 University Computing Service

   167 Cambridge CB2 3QH, England.

   168 .fi

   169 .

   170 .

   171 .SH REVISION

   172 .rs

   173 .sp

   174 .nf

   175 Last updated: 29 May 2007

   176 Copyright (c) 1997-2007 University of Cambridge.

   177 .fi