{ my @matches; my $push = qr/(?{ push @matches, $1 })/; sub match_all_ways { my ($string, $regex) = @_; @matches = (); $string =~ m/($regex)$push(?!)/; return @matches; } } sub common_substr { my ($str1, $str2, $len) = @_; my %substr = map { $_ => 1 } match_all_ways($str1 => qr/.{$len}/); $substr{$_} |= 2 for match_all_ways($str2 => qr/.{$len}/); return grep { $substr{$_} == 3 } keys %substr; } print "$_\n" for common_substr("ABCDEF", "ABDEFCBDEAB", 2); __END__ DE EF AB