If $c is actually a pattern, you should use the lookbehind. If you're just dealing with strings, the fastest might be some work with substr/index. 
    use Benchmark;
    use vars qw/$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  },
        substr     => sub {
            local $_ = "foobarquux";
            while ((my $p = index($_, "$c$a"))>=0) {
                substr($_, $p+length($c), length($a), $b);
            }
        },
    });

[download]
Output:
 chromatic: 11 wallclock secs (10.67 usr +  0.00 sys = 10.67 CPU) @ 46
+860.36/s (n=500000)
chromatic2: 10 wallclock secs ( 8.81 usr +  0.00 sys =  8.81 CPU) @ 56
+753.69/s (n=500000)
lookbehind: 10 wallclock secs ( 9.42 usr +  0.00 sys =  9.42 CPU) @ 53
+078.56/s (n=500000)
    substr:  4 wallclock secs ( 3.98 usr +  0.00 sys =  3.98 CPU) @ 12
+5628.14/s (n=500000)

[download]

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

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