Description: 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.

use warnings;
use strict;

while (<>) {
    print "fibo\n" if /^\s*(?:((?(1)(?(3)(?(2)\2|\3)(\1)|(\S))|\S))\s+
Replies are listed 'Best First'.
Re: fibonacci regex
by TedYoung (Deacon) on Jan 25, 2005 at 15:39 UTC

    Nice, this reminds me of a function to test if a number if prime:

    sub isPrime { ('1' x $_[0]) !~ /^(11+)\1+$/; }

    Ted Young

    ($$<<$$=>$$<=>$$<=$$>>$$) always returns 1. :-)