I'm currently working on a ruby gem to generate examples of given regular expression. For example:

    /this|is|awesome/.examples # => ["this", "is", "awesome"]

So, I tried it out on this "prime number validator" regex:

    /1?|(11+?)\1+/.examples
      #=> ["", "1", "1111", "111111", "11111111"]
    /1?|(11+?)\1+/.examples(max_repeater_variance: 50, max_group_results: 100).uniq.sort.map(&:length).first(10)
      #=> [0, 1, 4, 6, 8, 9, 10, 12, 14, 15]

    require 'prime'
    Array(1..100) - /1?|(11+?)\1+/.examples(max_repeater_variance: 50, max_group_results: 3000).uniq.sort.map(&:length) == Prime.first(25)
      #=> true

Horribly inefficient, but pretty cool! Here's my gem if case anyone's interested: https://github.com/tom-lord/regexp-examples