Creating and maintaining regular expressions are not like riding a bike. You can create a regular expression for a project and come back a couple months later and it is unreadable. Scott Hanselman has a saying, "If you try to solve a problem with a regular expression, you now have two problems."
Fortunately there are several resources available for free to help you test and understand regular expressions.
The Kellerman Quick Reference Pack that contains a Regular Expression cheat sheet:
Free Quick Reference Pack
If you work at a bank and can't install software use Reggie. This is a regular expression tester, and reference in a simple HTML file. This is my favorite.
If you are just getting started with Regular Expressions, Roy Osherove has a great free tool called Regulazy that will help you build simple Regular Expressions.
Roy also has an advanced tool (also free) called Regulator for building regular expressions.
Someone may have already built the regular expression you need. Take a look at this giant repository of regular expressions.