^(11+?)\1+$/ that can be used to check if a number is prime. The regular expression works by generating a string of 1s based on the number and checking if that string matches the pattern. The pattern matches either 1 or an empty string, or a string with repeating blocks of 11 or more 1s. If the string of 1s for a number does not match the pattern, then the number is prime. The author provides ex..