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Answer: How do i find $a and replace it with $b when it's preceded by $c?

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Q&A > regular expressions > How do i find $a and replace it with $b when it's preceded by $c? contributed by Roy Johnson

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); } }, });
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)

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