diff options
Diffstat (limited to 'pod/perlrebackslash.pod')
| -rw-r--r-- | pod/perlrebackslash.pod | 58 |
1 files changed, 34 insertions, 24 deletions
diff --git a/pod/perlrebackslash.pod b/pod/perlrebackslash.pod index 6587ea97d6..5e514ceec6 100644 --- a/pod/perlrebackslash.pod +++ b/pod/perlrebackslash.pod @@ -360,41 +360,51 @@ absolutely, relatively, and by name. =head3 Absolute referencing -A backslash sequence that starts with a backslash and is followed by a -number is an absolute reference (but be aware of the caveat mentioned above). -If the number is I<N>, it refers to the Nth set of parentheses - whatever -has been matched by that set of parenthesis has to be matched by the C<\N> -as well. +Either C<\gI<N>> (starting in Perl 5.10.0), or C<\I<N>> (old-style) where I<N> +is an positive (unsigned) decimal number of any length is an absolute reference +to a capturing group. + +I<N> refers to the Nth set of parentheses - or more accurately, whatever has +been matched by that set of parenthesis. Thus C<\g1> refers to the first +capture group in the regex. + +The C<\gI<N>> form can be equivalently written as C<\g{I<N>}> +which avoids ambiguity when building a regex by concatenating shorter +strings. Otherwise if you had a regex C</$a$b/>, and C<$a> contained C<"\g1">, +and C<$b> contained C<"37">, you would get C</\g137/> which is probably not +what you intended. + +In the C<\I<N>> form, I<N> must not begin with a "0", and there must be at +least I<N> capturing groups, or else I<N> will be considered an octal escape +(but something like C<\18> is the same as C<\0018>, that is the octal escape +C<"\001"> followed by a literal digit C<"8">). + +Mnemonic: I<g>roup. =head4 Examples - /(\w+) \1/; # Finds a duplicated word, (e.g. "cat cat"). - /(.)(.)\2\1/; # Match a four letter palindrome (e.g. "ABBA"). + /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat"). + /(\w+) \1/; # Same thing; written old-style + /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA"). =head3 Relative referencing -New in perl 5.10.0 is a different way of referring to capture buffers: C<\g>. -C<\g> takes a number as argument, with the number in curly braces (the -braces are optional). If the number (N) does not have a sign, it's a reference -to the Nth capture group (so C<\g{2}> is equivalent to C<\2> - except that -C<\g> always refers to a capture group and will never be seen as an octal -escape). If the number is negative, the reference is relative, referring to -the Nth group before the C<\g{-N}>. +C<\g-I<N>> (starting in Perl 5.10.0) is used for relative addressing. (It can +be written as C<\g{-I<N>>.) It refers to the I<N>th group before the +C<\g{-I<N>}>. -The big advantage of C<\g{-N}> is that it makes it much easier to write +The big advantage of this form is that it makes it much easier to write patterns with references that can be interpolated in larger patterns, even if the larger pattern also contains capture groups. -Mnemonic: I<g>roup. - =head4 Examples - /(A) # Buffer 1 - ( # Buffer 2 - (B) # Buffer 3 - \g{-1} # Refers to buffer 3 (B) - \g{-3} # Refers to buffer 1 (A) + /(A) # Group 1 + ( # Group 2 + (B) # Group 3 + \g{-1} # Refers to group 3 (B) + \g{-3} # Refers to group 1 (A) ) /x; # Matches "ABBA". @@ -403,9 +413,9 @@ Mnemonic: I<g>roup. =head3 Named referencing -Also new in perl 5.10.0 is the use of named capture buffers, which can be +Also new in perl 5.10.0 is the use of named capture groups, which can be referred to by name. This is done with C<\g{name}>, which is a -backreference to the capture buffer with the name I<name>. +backreference to the capture group with the name I<name>. To be compatible with .Net regular expressions, C<\g{name}> may also be written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>. |