Regular Expression Tutorial - Learn How to Use Regular Expressions
http://www.regular-expressions.info/tutorial.html

RegularExpressions.info

Abstract:

Basically, a regular expression is a pattern describing a certain amount of text. Their name comes from the mathematical theory on which they are based. In this tutorial, I will teach you all you need to know to be able to craft powerful time-saving regular expressions. I will start with the most basic concepts, so that you can follow this tutorial even if you know nothing at all about regular expressions yet.