<Editor's Note:>
Here is chromatic's post to which btrott is responding.

I prefer a simpler approach:

s/$c$a/$c$b/g;

or

s/($c)$a/$1$b/g;

[download]
Am I missing anything the lookbehind provides?

</Editor's Note>

All right... here's a benchmark of the two methods (lookbehind and just straight substitution). It looks like lookbehind is slightly faster, but not by a huge lot. Here's the code: 

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

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

[download]
And here are the benchmark results: 
    Benchmark: timing 500000 iterations of chromatic, lookbehind...
    chromatic: 15 wallclock secs (14.46 usr +  0.00 sys = 14.46 CPU)
    lookbehind: 13 wallclock secs (11.93 usr +  0.00 sys = 11.93 CPU)

[download]

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

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.