Regular expression that checks for primes.

This via

Noulakaz

It might well be, but it’s a hack as it bombs at larger numbers:

irb(main):001:0> def isp(n)

irb(main):002:1> (“1” * n) !~ /^1?$|^(11+?)\1+$/

irb(main):003:1> end

=> nil

irb(main):004:0> isp(55)

=> false

irb(main):005:0> isp(57)

=> false

irb(main):006:0> isp(101)

=> true

irb(main):007:0> isp(100000001)

RegexpError: Stack overflow in regexp matcher: /^1?$|^(11+?)\1+$/

from (irb):2:in `isp’

from (irb):7

from :0

irb(main):001:0> def isp(n)irb(main):002:1> (“1” * n) !~ /^1?$|^(11+?)\1+$/irb(main):003:1> end=> nilirb(main):004:0> isp(55)=> falseirb(main):005:0> isp(57)=> falseirb(main):006:0> isp(101)=> trueirb(main):007:0> isp(100000001)RegexpError: Stack overflow in regexp matcher: /^1?$|^(11+?)\1+$/        from (irb):2:in `isp’        from (irb):7        from :0

Yes regular expressions are cool and all but they are hacks.