This is Perl, so there's more than one way of doing this. One is indeed a regexp:

$ext="i_am_the_walrus.txt";
$ext=~s/^[^.]*(\.[\w]+$)/$1/;
print $ext;
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A second way is to use substr and index:

use strict;
my $filename="you_are_the_whale.doc";
my $separator=".";
unless (($where=index($filename,$seperator) == -1) {
  my $ext=substr($filename,$where);
  print $ext;
} else {
  print "No extensions in the zoo.\n"; }
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The third, and probably nicest way, is to use File::Basename:

use strict;
use File::Basename;
my $filename="we_are_not_all_mammals.zoo";
my ($base, $directory, $ext)=fileparse($filename, '\.[\w]+$');
print $ext;
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Beware that the above assumes you're dealing with simple .xyz extensions. For more complicated stuff, see the regexp in elusion's reply above.

CU
Robartes-

Update: Fixed regexp so it actually works :) (thx elusion). Adapted second recipe to include dot.