Ruby isPrime Method

Question

('1' * N) !~ /^1?$|^(11+?)\1+$/

On the net, I found this piece of Ruby code that works for N >= 0 that determines whether or not N is a prime. From what I can tell, it looks like play with regex but I have no idea how it works. Could someone tell me how it works?

Is this really an indictment of Ruby's obscure syntax? If so, I totally agree - wow is that obscure! — S.Lott, Sep 29 '08 at 01:50
What do you mean by "Ruby's obscure syntax"? Regexps look pretty much the same in all languages, don't they? — Jörg W Mittag, Sep 29 '08 at 01:54
This is just an obscure regex, it's not actually anything to do with ruby — Orion Edwards, Sep 29 '08 at 01:56
wow! I'm impressed, really. but this will work because this is POSIX regex. Normally we cannot build deterministic finite state machine that will accept/reject primary number and as I know DFA <=> regex. So this is not property of regular expression but of the posix regular expression — dfens, Aug 23 '10 at 14:11

score 25 · Accepted Answer · answered Sep 29 '08 at 01:53

25

You can find a lengthy explanation of this code here: http://www.noulakaz.net/weblog/2007/03/18/a-regular-expression-to-check-for-prime-numbers/

answered Sep 29 '08 at 01:53

Jay

41,768
14
66
83

Thanks. I am basking in the glow if it came from a Perl programmer first. – J.J. Sep 29 '08 at 02:37

score 21 · Answer 2 · answered Jan 18 '10 at 19:58

21

This is probably rather off-topic, but in Ruby 1.9, you can do this:

 require 'mathn'
 38749711234868463.prime?
 => false

answered Jan 18 '10 at 19:58

Trevoke

4,115
1
27
48

score 4 · Answer 3 · edited Apr 01 '14 at 08:46

4

require 'prime'

Prime.prime?(4)
# => false

Prime.prime?(5)
# => true

Or:

require 'prime'

Prime.instance.prime?(4)
# => false

Prime.instance.prime?(5)
# => true

edited Apr 01 '14 at 08:46

toro2k

19,020
7
64
71

answered Apr 01 '14 at 08:44

rajagopal

49
4

score 2 · Answer 4 · edited May 23 '17 at 12:26

2

See also What is the most brilliant regex you’ve ever used? (and yes, I can confirm that this regexp was originally written by Abigail. I've even heard her explain how it works :)

edited May 23 '17 at 12:26

Community

1
1

answered Sep 29 '08 at 07:55

dland

4,319
6
36
60

score 2 · Answer 5 · answered Nov 13 '10 at 20:52

If the length of a string of 1's is composite, then the string can be decomposed into multiple identical substrings, like 111111 -> 11 11 11

For example, 1111111111, has 10 1's, and it matches (11){5} or (11111){2}, where {2} means repeated 2 times. 111111111, has 9 1's, and it matches (111){3}.

By generalizing the count of 1's and the number in {}, the regexp is /(1{2,}){2,}/. However, 1{2,} can also be written as 11+, and (...){2,} can be rewritten as (...)\1+, with backreferences.

The ^1?$ part in the first alternation checks for 0 and 1-cases.

score 1 · Answer 6 · answered Sep 29 '08 at 07:42

Greatest Common Divisor (gcd):

/^(1+)\1*=\1+$/.match('1' * x + '=' + '1' * y)[1].length

Both this and the is_prime one works in about the same way. It tries all combinations before giving up.

This one will try to split the first number in even parts, and match the second number with one or more of those parts. If it finds a match it returns the length of the selected part.

score 1 · Answer 7 · answered Jan 18 '10 at 20:35

1

Yet another blog with a pretty good explanation: Famous Perl One-Liners Explained (part III)

answered Jan 18 '10 at 20:35

MAK

26,140
11
55
86

Ruby isPrime Method

7 Answers7