Regexp finding Prime numbers
Tue 2010-01-05 ( En pr ) 
(1..100).select { |n| p n if ('1' * n) !~ /^1?$|^(11+?)\1+$/ }

Basically, the /^(11+?)\1+$/ part checks if the 1-string can be factorized. See Avinash Meetoo.

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