Detect Prime Numbers with a Regex -- Source: https://iluxonchik.github.io/regular-expression-check-if-number-is-prime/

prime_re.rb

class Fixnum
  def prime?
    /^.?$|^(..+?)\1+$/ !~ '1'*self
  end
end

0.prime? #=> false
1.prime? #=> false
2.prime? #=> true
3.prime? #=> true
4.prime? #=> false
5.prime? #=> true
6.prime? #=> false
7.prime? #=> true
8.prime? #=> false
9.prime? #=> false