||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:
x x xx xxx xxxxx
Ok, it's probably not very useful, but I thought I'd post it anyway.