#!/usr/bin/perl

# http://perlmonks.org/?node_id=1207241

use strict;
use warnings;

my $specific = 'right';

while(<DATA>)
  {
  chomp;
  /(?|(.*?)($specific.*)|(.*)())/;
  printf "%-12s  \\1=%4s  \\2=%s\n", $_, $1, $2;
  }

__DATA__
left
right
rightabc
leftright
leftrightabc
[download]

Outputs:

left          \1=left  \2=
right         \1=      \2=right
rightabc      \1=      \2=rightabc
leftright     \1=left  \2=right
leftrightabc  \1=left  \2=rightabc
[download]