About eight years ago I finally read Friedl's Mastering Regular Expressions [1]. I know, right? A 500-page book about regular expressions, a tool I already knew (or thought I did). But it's actually a great book-- easy to read and full of genuinely good information on the how and why of regex, and it totally changed my understanding of them. If absolutely anything in this article surprised you, I highly recommend you read the book.

[1] http://regex.info/book.html