See your regular expressions in Emacs
∞First, if you are an Emacs newbie then be sure to read (at least) the introduction of Being Productive with Emacs. For some reason the PDF and HTML versions are slightly similar.
Anyway, it mentions re-builder which is an awesome little gem if you use regular expressions at all1. What this baby does is open a small window at the bottom of your screen in which you can type a regex. It is parsed as you type it and matches are highlighted in the other window. Genius.
[1] If you don't use them I encourage you to "learn them"http://regex.info/. Don't pay any attention to Jamie Zawinsky and his lack of appreciation for a fantastic tool.