If you install perl, you can type
perldoc perlre 
to learn something about regular expression.
It's a little different bewteen perlre and 
POSIX regexp which Ruby used.
You also can get some info from 
info libc about regexp.

andyu
Chris Morris <chrismo / homemail.com> wrote:
>Date: Tue, 13 Feb 2001 12:09:57 +0900
>From: "Chris Morris" <chrismo / homemail.com>
>Subject: [ruby-talk:10764] Online regular expression reference?
>To: ruby-talk / ruby-lang.org (ruby-talk ML),
>        "Ruby-Talk" <ruby-talk / netlab.co.jp>
>
>Is there a handy regular expression reference online anywhere?
>
>Chris
>