[bantic]

The thing that made this click for me was realizing the "1" in the regex was not special. The regex is just looping through integers (starting with 2) and checking if the target number is divisible by any of them. It can use any digit if the target string uses the same digit. Ruby example using 5 instead of 1:

def is_prime(n); str = "5"*n; str !~ /^5?$|^(55+?)\1+$/; end

The regex essentially says, for integer n, "does 2 ones fit evenly into n ones", then "does 3 ones fit into n ones", etc., which is the same as "does 2 fit into n", "does 3 fit into n", etc.