#!/usr/bin/perl

use strict;
use warnings;

$_ = "---abc";

/(?=abc)\b/  and print  "Lookahead \$` = '$`'; \$' = '$''\n";
/(?<=abc)\b/ and print "Lookbehind \$` = '$`'; \$' = '$''\n";

__END__
Lookahead $` = '---'; $' = 'abc'
Lookbehind $` = '---abc'; $' = ''