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.
Articles:
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.