performance improvement in method to strip non-numeral characters from string ??

I need to write a method such that when we pass on a string to it, it
returns a substring out of it. returned string is such that it
consists of numerals only. All the characters other than numerals are
stripped away.

e.g.
" 2009-05-30 10:25:15 UTC" ==> 20090530102515

I have written the following method for the same.
It seems to be working but I am looking for your suggestions /
feedback for
  --> performance improvment
  --> any bug that i am not able to see

def getNumeralString(istr)
    x = istr.split(//)
    istr = ""
    x.each{|a|
      if a[0] >= '0'[0] && a[0] <= '9'[0]
        istr = istr +a
      end
    };
  return istr
end

Thanks for feedback

I need to write a method such that when we pass on a string to it, it
returns a substring out of it. returned string is such that it
consists of numerals only. All the characters other than numerals are
stripped away.

e.g.
" 2009-05-30 10:25:15 UTC" ==> 20090530102515

s.gsub(/[^0-9]/,'')

is more concise and probably faster

Fred

I think Fred's nailed the solution already, but some more general
advice:

- I'd *highly* recommend you read Programming Ruby (the Pickaxe book),
either online (http://www.rubycentral.com/book/), or purchased from
Pragmatic Programmers (http://www.pragprog.com/titles/ruby/programming-
ruby). Your original code looks like a classic example of "C
programmers can program any language in C", and learning the Ruby
idioms will help avoid this sort of thing.

- I'd also recommend that you work on relaxing your "speed optimizing"
tendencies. Skimming through some of your other posts to various
Google Groups shows quite a few discussions most would describe as
"premature optimization". While it's important to be a little
concerned about performance (ie, not writing algorithms that are
deliberately inefficient), it's equally destructive to be too
concerned about performance. At least at the outset, just write clear
code. A surprising amount of the time in Ruby, clear code (Fred's
solution using gsub) is significantly faster than unclear code.

--Matt Jones

Thanks for advice, Matt. I would like to read that book as soon as i
get the time from current project.
Thanks
vipin