To summarize what's been done by chromatic, btrott and nuance, with the chromatic version using no backreferences named chromatic2, we have: 
Benchmark: timing 500000 iterations of chromatic, chromatic2, lookbehi
+nd...
 chromatic: 18 wallclock secs (18.83 usr + 0.01 sys = 18.84 CPU) @ 265
+43.50/s
chromatic2: 13 wallclock secs (13.55 usr + 0.00 sys = 13.55 CPU) @ 369
+00.37/s
lookbehind: 14 wallclock secs (14.95 usr + 0.00 sys = 14.95 CPU) @ 334
+42.58/s

[download]
for the test: 
    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  }
+,
    });

[download]

In reply to Re: How do i find $a and replace it with $b when it's preceded by $c? by Incognito
in thread How do i find $a and replace it with $b when it's preceded by $c? by whahoo

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