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.
Link: PHPro