Writing your own regex library can help you understand some of the more obscure features of Perl's regexes. :-)
The following regex matches lines consisting of words whose length form the fibonacci sequence 1, 1, 2, 3, 5, 8, 13, etc. Caveat: All words must consist of the same character.

Examples of valid input:
v
x x xx xxx xxxxx

Ok, it's probably not very useful, but I thought I'd post it anyway.

#!/usr/local/bin/perl
use warnings;
use strict;

while (<>) {
    print "fibo\n" if /^\s*(?:((?(1)(?(3)(?(2)\2|\3)(\1)|(\S))|\S))\s+
+)+\z/;
}
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