An excellent article, kudos to the writer for doing it in a programmer centered way. I liked the way it was structured. It is self-explanatory and doesn't assume that the reader is a dummy. Wish more articles were presented in this fashion, especially the references (javadoc, I am pointing to you)
http://pypix.com/tools-and-tips/advanced-regular-expression-tips-techniques/
http://pypix.com/tools-and-tips/advanced-regular-expression-tips-techniques/