It's basically the Sieve of Eratosthenes[1] algorithm, and it's possible to do it with regular expressions because numbers here are represented in unary[2] numb

It's basically the Sieve of Eratosthenes[1] algorithm, and it's possible to do it with regular expressions because numbers here are represented in unary[2] number system, where number of characters/tokens equals the number itself. It's a common trick for testing various Turing-machine stuff. [1] https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes [2] https://en.wikipedia.org/wiki/Unary_numeral_system https://en.wikipedia.org/wiki/Unary_numeral_system