deepsoul said on 02/Aug/2009

Sun, 02 Aug 2009 17:55:24 UTC

The test works on unary notation, not decimal. So 2 is written as "11", 3 as "111", 17 as "11111111111111111" and so on. The first branch of the regex matches the empty string (representing 0) and "1" (representing 1), marking them as non-primes. The second branch succeeds if and only if the unary number consists of two or more times two or more 1s, which means it is the product of these two numbers (both at least 2), so not a prime. To print a string of primes and see that this works: perl -l -e '(1 x $_) !~ /^1?$|^(11+?)\1+$/ & print while ++$_;' See also: http://www.catb.org/jargon/html/O/one-liner-wars.html

Comments on snippet: 'Prime number tester'

deepsoul said on 02/Aug/2009