I would nominate two regexes that merlyn was responsible for, one being the answer to "I want to replace spaces with underscores, but only where they're found between brackets" on a newsgroup posting somewhere.

It's a relatively simple one, but it opened my eyes, as a beginner, to a whole world of nested and executing regexes.

I'm reproducing it here from memory so merlyn will forgive me if it's not quite how he did it:

$str= 'no change <these spaces need replacing> not these <these do>';
$str =~ s{(<[^>]*?>)}
        {
         my $x=$1;
         $x=~s/ /_/g;
         $x;
        }egx;
print $str
[download]

And the other one I can't remember at all, but I remember it involved Old MacDonald, and a regex that double-executed, and therefore ended "/eieio". Has to be included.
--

($_='jjjuuusssttt annootthheer
     pppeeerrrlll haaaccckkeer')=~y/a-z//s;print;
[download]