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comp.lang.ruby

Regexp and Prime numbers

Tim Pease

3/19/2007 7:07:00 PM

This is a one-liner Ruby script that will tell you if a given number is prime.

ruby -wle 'puts "Prime" unless ("1" * ARGV[0].to_i) =~
/^1$|^(11+?)\1+$/' [number]


I cannot take credit for this one -- it originally came from Perl. The
website listed below gives the full explanation of how the regexp
works.

http://montreal.pm.org/tech/neil_kandalgao...

I highly recommend reading this if you want to flex your regexp muscles today.

Blessings,
TwP


ruby -wle 'puts "Prime" unless ("1" * ARGV[0].to_i) =~ /^1$|^(11+?)\1+$/' 42

It may be the answer to life, the universe, and everything, but it
certainly isn't prime!
3 Answers

Tim Pease

3/20/2007 2:18:00 PM

0

On 3/20/07, gga <GGarramuno@aol.com> wrote:
> On Mar 19, 4:07 pm, "Tim Pease" <tim.pe...@gmail.com> wrote:
> >
> > ruby -wle 'puts "Prime" unless ("1" * ARGV[0].to_i) =~ /^1$|^(11+?)\1+$/' 42
> >
>
> This is pretty cool. Particularly as a simple benchmark for regex
> backtracking.
> It highlites some of the performance issues of backtracking in ruby1.8
> and 1.9.
> And it also highlites issues with Perl's backtracking.
>
> Speed issues:
> > time ruby1.9 -wle 'puts "Prime" unless ("1" * ARGV[0].to_i) =~ /^1$|^(11+?)\1+$/' 22441
> Prime
>
> real 0m2.111s
> user 0m2.084s
> sys 0m0.024s
>
> > time perl -wle 'print "Prime" if (1 x shift) !~ /^1$|^(11+?)\1+$/' 22441
> Prime
>
> real 0m0.230s
> user 0m0.208s
> sys 0m0.020s
>
>
> But... on larger numbers...
>
> > ruby1.9 -wle 'puts "Prime" unless ("1" * ARGV[0].to_i) =~ /^1$|^(11+?)\1+$/' 104729
> Prime
>
> > perl -wle 'print "Prime" if (1 x shift) !~ /^1$|^(11+?)\1+$/' 104729
> Segmentation fault
>
> > perl -v
> This is perl, v5.8.8 built for x86_64-linux-gnu-thread-multi
>

Now that is very interesting. Do you know if Ruby 1.9 is running
Onigurma <sp?>, the new regexp handler?

James Gray

3/26/2007 5:45:00 PM

0

On Mar 19, 2007, at 2:07 PM, Tim Pease wrote:

> This is a one-liner Ruby script that will tell you if a given
> number is prime.
>
> ruby -wle 'puts "Prime" unless ("1" * ARGV[0].to_i) =~
> /^1$|^(11+?)\1+$/' [number]
>
>
> I cannot take credit for this one -- it originally came from Perl. The
> website listed below gives the full explanation of how the regexp
> works.
>
> http://montreal.pm.org/tech/neil_kandalgao...
>
> I highly recommend reading this if you want to flex your regexp
> muscles today.

Wow, that was just awesome.

James Edward Gray II

email55555 email55555

4/24/2007 7:49:00 PM

0

Cool! I couldn't resist porting it to Java, so here is my Java version
(It is not one-liner, but maybe one statement?) :

public class PrimeTester {
public static void main(String[] args) {
System.out.println(String.format("%0" + args[0] + "d",
0).matches("^0$|^(00+?)\\1+$") ? "Not prime" : "Prime");
}
}

http://davidtran.doublegifts.com/...


On Mar 19, 3:07 pm, "Tim Pease" <tim.pe...@gmail.com> wrote:
> This is a one-liner Ruby script that will tell you if a given number isprime.
>
> ruby -wle 'puts "Prime" unless ("1" * ARGV[0].to_i) =~
> /^1$|^(11+?)\1+$/' [number]
>
> I cannot take credit for this one -- it originally came from Perl. The
> website listed below gives the full explanation of how theregexp
> works.
>
> http://montreal.pm.org/tech/neil_kandalgao...
>
> I highly recommend reading this if you want to flex yourregexpmuscles today.
>
> Blessings,
> TwP
>
> ruby -wle 'puts "Prime" unless ("1" * ARGV[0].to_i) =~ /^1$|^(11+?)\1+$/' 42
>
> It may be the answer to life, the universe, and everything, but it
> certainly isn'tprime!