NETTUTS.com has posted an advanced guide to regular expressions that just happens to use the PHP PCRE functions to do the matching.

Regular Expressions are the Swiss Army knife for searching through information for certain patterns. They have a wide arsenal of tools, some of which often go undiscovered or underutilized. Today I will show you some advanced tips for working with regular expressions.

They have it broken out into several tips including:

• Using Callbacks
• Lookahead and Lookbehind Assertions
• Filtering Patterns
• Named Subpatterns

There's also a section called "Don’t Reinvent the Wheel" with a few handy expressions to do common things like parse [X]HTML and validating form input.