Grouping and 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

 Live Demo

#!/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

 Live Demo

#!/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