A regular expression to check for prime numbers

Previous discussion:

http://news.ycombinator.com/item?id=1486158

http://news.ycombinator.com/item?id=707236

Not a true regular language (by the pumping lemma), and the reason you should care is regular languages can be made very fast: http://swtch.com/~rsc/regexp/regexp1.html

Three words: only in Perl.