/[pcre]/code/trunk/doc/pcrepattern.3
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revision 172 by ph10, Tue Jun 5 10:40:13 2007 UTC revision 175 by ph10, Mon Jun 11 13:38:38 2007 UTC
# Line 958  is reached, an option setting in one bra Line 958  is reached, an option setting in one bra
958  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
959  .  .
960  .  .
961    .SH "DUPLICATE SUBPATTERN NUMBERS"
962    .rs
963    .sp
964    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
965    the same numbers for its capturing parentheses. Such a subpattern starts with
966    (?| and is itself a non-capturing subpattern. For example, consider this
967    pattern:
968    .sp
969      (?|(Sat)ur|(Sun))day
970    .sp
971    Because the two alternatives are inside a (?| group, both sets of capturing
972    parentheses are numbered one. Thus, when the pattern matches, you can look
973    at captured substring number one, whichever alternative matched. This construct
974    is useful when you want to capture part, but not all, of one of a number of
975    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
976    number is reset at the start of each branch. The numbers of any capturing
977    buffers that follow the subpattern start after the highest number used in any
978    branch. The following example is taken from the Perl documentation.
979    The numbers underneath show in which buffer the captured content will be
980    stored.
981    .sp
982      # before  ---------------branch-reset----------- after
983      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
984      # 1            2         2  3        2     3     4
985    .sp
986    A backreference or a recursive call to a numbered subpattern always refers to
987    the first one in the pattern with the given number.
988    .P
989    An alternative approach to using this "branch reset" feature is to use
990    duplicate named subpatterns, as described in the next section.
991    .
992    .
993  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
994  .rs  .rs
995  .sp  .sp
# Line 1007  abbreviation. This pattern (ignoring the Line 1039  abbreviation. This pattern (ignoring the
1039    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1040  .sp  .sp
1041  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1042    (An alternative way of solving this problem is to use a "branch reset"
1043    subpattern, as described in the previous section.)
1044    .P
1045  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1046  for the first (and in this example, the only) subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1047  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was. If you
# Line 1898  Cambridge CB2 3QH, England. Line 1933  Cambridge CB2 3QH, England.
1933  .rs  .rs
1934  .sp  .sp
1935  .nf  .nf
1936  Last updated: 29 May 2007  Last updated: 11 June 2007
1937  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2007 University of Cambridge.
1938  .fi  .fi
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