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Grouping Matching in Perl
From a regular-expression point of view in Perl, there is no difference between the following two expressions except that the former is slightly clearer.
$string =~ /(\S+)\s+(\S+)/; and $string =~ /\S+\s+\S+/;
However, the benefit of grouping is that it allows us to extract a sequence from a regular expression. Groupings are returned as a list in the order in which they appear in the original. For example, in the following fragment we have pulled out the hours, minutes, and seconds from a string.
my ($hours, $minutes, $seconds) = ($time =~ m/(\d+):(\d+):(\d+)/);
As well as this direct method, matched groups are also available within the special $x variables, where x is the number of the group within the regular expression. We could therefore rewrite the preceding example as follows −
Example
#!/usr/bin/perl $time = "12:05:30"; $time =~ m/(\d+):(\d+):(\d+)/; my ($hours, $minutes, $seconds) = ($1, $2, $3); print "Hours : $hours, Minutes: $minutes, Second: $seconds\n";
When above program is executed, it produces the following result −
Hours : 12, Minutes: 05, Second: 30
When groups are used in substitution expressions, the $x syntax can be used in the replacement text. Thus, we could reformat a date string using this −
Example
#!/usr/bin/perl $date = '03/26/1999'; $date =~ s#(\d+)/(\d+)/(\d+)#$3/$1/$2#; print "$date\n";
When above program is executed, it produces the following result −
1999/03/26
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