While regular expressions are most commonly used for text manipulation, such as searching and search-and-replace, they can also be used to test for certain conditions in a text file or data stream. The usefulness of this functionality?especially as it pertains to Web service development?is fairly obvious. These articles cover the basics.
Advanced String Handling with Regular Expressions
Mastering Regular Expressions
Using Regular Expressions
A Tao of Regular Expressions
Regular Expression HOWTO
IBM DeveloperWorks Regular Expressions Tutorial (free, but requires registration)
Book:Mastering Regular Expressions, by Jeffrey E.F. Friedl.