Regex primality test

prime.rb

# REGEX PRIMALITY TEST (after a perl by abigail.be)
# Call with int arg >0, e.g. $ ruby prime.rb 691

N = ARGV[0].to_i
print "#{N} is "
puts ('I'*N) !~ /^I?$|^(II+?)\1+$/ ? 'prime' : 'composite'

Adapted from Abigail’s perl.
Very inefficient, but craftily concise!
Call with +ve integer as argument, e.g. $ ruby prime.rb 691