Mike Malone has an excellent and comprehensive discussion on expressions. If you use Excel or Access, you're already familiar with expressions, but don't bypass Mike's discussions just because you think you know enough—there’s plenty of insight to go around. Anyone using VBA to automate any of the Office applications will benefit from Mike’s discussion at http://immike.net/blog/2007/04/06/the-absolute-bare-minimum-every-programmer-should-know-about-regular-expressions/
Thanks to Mike Gunderloy of Larkware, http://larkware.com/, for the heads up on this terrific article. Mike manages a blog for serious developers and programmers, but occasionally there's something good for the rest of us more casual users.