Noupe.com has posted a new guide to getting started with regular expressions in PHP. Regular expressions can be one of the most powerful tools at your disposal when used correctly.
The main purpose of regular expressions, also called regex or regexp, is to efficiently search for patterns in a given text. These search patterns are written using a special format which a regular expression parser understands.
They cover some of the basics - special characters and some examples using them (like $, d, and *) - before getting into the preg_* functions in PHP. They also include some handy examples to validate things like US zip codes, email format validation and removing duplicated words in a section of text.