Arcane isPrime method in Java

Question

Consider the following method:

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

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Answer 1

This is not a really efficient way to check if a number is prime(it checks every divisor).

An efficient way is to check for divisors up to sqrt(number). This is if you want to be certain if a number is prime. Otherwise there are probabilistic primality checks which are faster, but not 100% correct.