In a post from JellyandCustard.com, there's an inside look at regular expressions in PHP - how to use them and what they are.
Regular Expressions (regex for short) appear to a lot of people as the 'black art' of coding. Most languages, be it PHP, Java, C, .NET, VB etc have a way of using regular expressions - and they can certainly make your job easier. So lets start on our journey into regular expressions. I am by no means an expert, but hopefully I'll be able to clear the fog that surrounds regular expressions!
He mentions the functions to use and provides a simple example of matching a phone number (US format). They take the regular expression string and break it down into its pieces, explaining how each one works (and relates to the other parts of the expression). They also provide a brief "cheat sheet" of some of the other special characters (including common uses on some).