http://news.ycombinator.com/item?id=230292
If your browser has Java applet support, here you can watch the regex succeed (demonstrating a number is composite) or fail (prime):
49: http://regex.powertoy.org/?pat=/^1%3F%24|^%2811+%3F%29\1+%24...
47: http://regex.powertoy.org/?pat=/^1%3F%24|^%2811+%3F%29\1+%24...
http://www.cpan.org/misc/japh
(Hmm... I'll use that as a fortune file. Just need to remove extra '%', if fortune barfs on them. Ah, haven't had fortune in my login since the 90s.)
http://news.ycombinator.com/item?id=230292
If your browser has Java applet support, here you can watch the regex succeed (demonstrating a number is composite) or fail (prime):
49: http://regex.powertoy.org/?pat=/^1%3F%24|^%2811+%3F%29\1+%24...
47: http://regex.powertoy.org/?pat=/^1%3F%24|^%2811+%3F%29\1+%24...