Alice, Bob, and Mallory: Find primes in regexp

Find primes in regexp

Jonas Elfström — Fri, 30 Mar 2007 07:33:00 +0200

In an earlier post the example code did find prime numbers. Recently I stumbled over a really cool regexp hack that also deals with primes. This is how you execute that regexp in Ruby:

puts 'Prime' unless ('1' * 43) =~ /^1$|^(11+?)\1+$/

Change 43 to whatever you like and you will get Prime as output if it's a prime number.

EDIT: As you can see in the comments Neil Kandalonkar explained how the regexp by Abigail works.

EDIT 2011-04-06: I happened to stumble upon what I believe is the first time Abigail showed this to the world. It's in a post in comp.lang.perl.misc back in 1997. I also found that at http://www.cpan.org/misc/japh there's a couple more

perl -wle 'print "Prime" if (0 x shift) !~ m 0^\0?$|^(\0\0+?)\1+$0'
perl -wle 'print "Prime" if ("m" x shift) !~ m m^\m?$|^(\m\m+?)\1+$mm'

"Find primes in regexp" by Jonas Elfström

Thu, 12 Jan 2012 12:59:13 +0100

Peteris Krumins explanation of how this works can be found at http://www.catonmat.net/blog/perl-regex-that-matches-prime-numbers/

"Find primes in regexp" by Jonas Elfström

Wed, 04 Aug 2010 09:44:05 +0200

The prime that wasn’t - explains how it works, it’s inefficiency and why PHP reported 22201 as prime.

The Ruby implementation of RegEx differs in that aspect but I don’t know if it has a set limit or if it’s just limited to virtual memory of the machine.

> ('1' * 222201) =~ /^1$|^(11+?)\1+$/
=> 0
> ('1' * 222199) =~ /^1$|^(11+?)\1+$/
=> nil

222201 is not prime but 222199 is and that is what the above shows. The second expression took a couple of minutes to run.

"Find primes in regexp" by Jonas Elfström

Mon, 18 Jan 2010 21:46:43 +0100

http://stackoverflow.com/questions/20448/what-is-the-most-brilliant-regex-youve-ever-used/20546#20546

And it was created by http://www.abigail.be/

"Find primes in regexp" by Jonas Elfström

Sun, 20 Sep 2009 23:07:14 +0200

Seems Neils site has gone missing. Here’s another nice explanation: http://paddy3118.blogspot.com/2009/08/story-of-regexp-and-primes.html

"Find primes in regexp" by Jonas Elfström

Mon, 28 May 2007 00:53:36 +0200

Sorry, now it has been corrected.

"Find primes in regexp" by Neil Kandalgaonkar

Sat, 26 May 2007 22:43:37 +0200

As the page makes clear, credit should go to Abigail. I merely explained it.