First, if you are an Emacs newbie then be sure to read (at least) the introduction of <a href="http://stuff.mit.edu/iap/emacs">Being Productive with Emacs</a>. For some reason the PDF and HTML versions are slightly similar.

Anyway, it mentions <code>re-builder</code> which is an awesome little gem if you use regular expressions at all<sup>1</sup>. 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 <a href="http://regex.info/blog/2006-09-15/247">Jamie Zawinsky</a> and his lack of appreciation for a fantastic tool.
