Benchmark: timing 500000 iterations of chromatic, chromatic2, lookbehind...
 chromatic: 18 wallclock secs (18.83 usr + 0.01 sys = 18.84 CPU) @ 26543.50/s
chromatic2: 13 wallclock secs (13.55 usr + 0.00 sys = 13.55 CPU) @ 36900.37/s
lookbehind: 14 wallclock secs (14.95 usr + 0.00 sys = 14.95 CPU) @ 33442.58/s


##</code><code>##

    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  },
    });