using CRYPT but restrict alpha-numeric

Hello.

I'm looking at the various CRYPT / one-way encryption commands and i
was wondering if there is a way to create a crypt but restrict output
to alpha-numeric (a to z, A to Z and 0-9) without the special
characters ?

Basically ive got a starting reference - numeric over 10,000,000 and
other numbers and want to do a crypt (or something) but exclude
special characters from the crypt result.

The best I can think of is to do a crypt (or whatever) etc and then do
a preg_replace to remove the special characters etc..

The result will be shorter (length), but will be an alpha-numeric crypt.

--

Gordon.