perl -wle 'print "Prime" if (1 x shift) !~ /^1?$|^(11+?)\1+$/' <number>

I saw that by Abigail on comp.lang.perl.misc many moons ago. Here is an article about it: http://test.neilk.net/blog/2000/06/01/abigails-regex-to-test...

As far as I know, she was the genesis of this whole thing.