To save a click, the regex in question is: ^1?$|^(11+?)\1+$ (it checks if a unary number is not prime. It is kind of surprising to hear that regex can do that,

To save a click, the regex in question is: ^1?$|^(11+?)\1+$ (it checks if a unary number is not prime. It is kind of surprising to hear that regex can do that, but once you see the regex its kind of obvious. Its just checking if the number is 1, or if the number can be represented by 2 or more 1's repeated at least 2 times. Which is literally the definition of a prime (is the number divisible by a number >= 2)