Additional resources
If reading this introduction to regular expressions got you interested in learning even more, then take a look at the following resources if you want to delve deeper into the subject:
The book Mastering Regular Expressions (O'Reilly) by Jeffrey E.F. Friedl covers the subject extremely thoroughly and is the definitive guide to the subject. At the time of this writing, the latest edition was the 3rd edition, released in 2006.
The web site http://www.regular-expressions.info/, which is maintained by Jan Goyvaerts (author of RegexBuddy) contains a tutorial on regular expressions, examples, and much more.
Jan Goyvaerts is also co-author of the book Regular Expressions Cookbok (O'Reilly, 2009), which is marketed as a source of practical examples of regular expressions as they are used in real life.