For those that are regex-challenged (like myself), determining exactly what a regex will match can be very much hit or miss. Recently, I stumbled across Rubular, which helps make this a little easier. Rubular is a Ruby regular expression editor, with a very sexy, ajaxy interface.
Type in your test string using the quick reference at the bottom of the page. Then enter your regular expression, and the observer takes over to show you what your regex matches on.
Very cool tool and one that I'm sure I'll be back to visit.