From: Michael Brooks <michael.brooks@...>
Date: 2009-02-24T11:17:45+09:00
Subject: 1.9.1 regex 6.5 times slower than 1.8.6 in at least one case

Hello:

In general I've found Ruby 1.9.1 to be 2 times faster than 1.8.6 and 
roughly the same speed as Python 2.8 which is great... thanks guys, 
great job!

However, in one case, when using a regular expressions (posted here by 
someone a long time ago) which I use to determine what numbers in 
0..10000 are prime numbers, version 1.9.1 was at least 6.5 times slower 
(1.8.6 = 67 secs, 1.9.1 = 457 secs).

The regular express is:

     ((("1" * self) =~ /^1$|^(11+?)\1+$/) == nil)

which I've used in different versions of my program running either on 
its own or as part of an overloading function as follows:

   # Add an "is_prime?" method to the built-in numeric class
   # which returns true if the number is a prime number.
   class Fixnum
     def is_prime?
       ((("1" * self) =~ /^1$|^(11+?)\1+$/) == nil)
     end
   end

I also have a version of the prime-number calculation program that which 
doesn't use the above regex (i.e. it uses a traditional brute force 
approach instead) and it runs 2 times faster in 1.9.1.  In 1.9.1 the 
regex gets exponentially worse as the number being evaluated exceeding 
1000.

Does anyone know why the regex in 1.9.1 is so much slower than 1.8.6?

Thank You,

Michael