If you enjoy reading about regular expressions, Cameron's paper is fascinating. His writing is concise, thorough, and very detailed. He's not simply showing you how to construct the REX regular expression but also an approach for constructing any complex regex.

I've been using the REX regular expression on and off for 10 years to solve the sort of problem the initial poster on Stack Overflow asked about (how do I match this particular tag but not some other very similar tag?). I've found the regex he developed to be completely reliable.