A while back I was researching the most efficient way to check if a number is prime. This lead me to find the following piece of code:

public static boolean isPrime(int n) {
    return !new String(new char[n]).matches(”.?|(..+?)\\1+”);

I was intrigued. While this might not be the most efficient way, it’s certainly one of the less obvious ones, so my curiosity kicked in. How on Earth could a match for the .?|(..+?)\1+ regular expression tell that a number is not prime (once it’s converted to its unary representation)?

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