use Benchmark;
    use vars qw/$a $b $c/;
    ($a, $b, $c) = qw/bar baz foo/;

    timethese(shift || 500000, {
        lookbehind => sub { local $_ = "foobarquux"; s/(?<=$c)$a/$b/g },
        chromatic  => sub { local $_ = "foobarquux"; s/($c)$a/$1$b/g  },
        chromatic2  => sub { local $_ = "foobarquux"; s/$c$a/$c$b/g  },
    });