Kevin Waterson has posted an extensive tutorial to his site today giving a great look at regular expressions in PHP (and just regular expressions in general).
At its most basic level, a regex can be considered a method of pattern matching or matching patterns within a string. In PHP the most oft used is PCRE or "Perl Compatible Regular Expressions". Here we will try to decypher the meaningless hieroglyphics and set you on your way to a powerful tool for use in your applications. Do not try to understand all this in a single sitting. Instead, take in a little and come back as you grasp various concepts.
He starts with simple string matching, showing how to get values from the middle, end and beginning of it then moves on to the next more powerful step - using meta characters. He outlines these as well as sequences and modifiers. There's plenty of examples here so there's almost no chance of getting lost.
It's a great tutorial that I'd recommend to anyone who things regular expressions are "just too hard" or it looking to figure out their power.