Over at smashing magazine, they have a great post on PHP’s preg_match() regular expressions.  They cover a number of advanced topics in light detail, including:

1. Greediness/Laziness

2. Back Referencing

3. Named Groups

4. Word Boundaries

5. Atomic Groups

6. Recursion

7. Callbacks

8. Commenting

See it here!

http://www.smashingmagazine.com/2009/05/06/introduction-to-advanced-regular-expressions/