On his web development blog, Joey has posted a new list of eight handy regular expressions you can use in your code for common validations.
Here are eight examples of practical PHP regular expressions and techniques that I've used over the past few years using Perl Compatible Regular Expressions. This guide goes over the eight different validation techniques and describes briefly how they work. Usernames, telephone numbers, email addresses, and more.
Each of the regular expressions comes complete with an explanation of what it is and the kinds of strings it's looking for. Comments on the post have helped even more, finding places where they might break or not catch all possibilities.