Regular Expressions are a very powerful tool for developers. The can be used for various jobs. A common use is for validation of user input agains a pattern. This can be done in code (using a method from the java.util.regex package) or e.g. with a JSF validator component (either home brew or ready made). Another use for regular expressions is doing advanced search-and-replace operations on (e.g.) source code. Most IDEs and the more advanced text editors offer search-and-replace based on regular expressions.
Whatever job you use regular expressions for, one thing is for sure: composing a regular expression is never easy. Therefore, testing a regular expression before using it is always a good idea. There are a lot of online regular expression testers out there. I tried some of the more advanced ones recently and thought it was a good idea to share my findings.