Ever programming against the .NET Framework and find you have to use the Regex class in the System.Text.RegularExpressions namespace? Ever wish you could load a file, enter in your regular expression, and maybe even your replacement expression, then see the output?
Me, I started using RadSoftware's free Design and Test Tool: http://www.radsoftware.com.au/regexdesigner/
It works quite nicely! I don't know if it would help with non-.NET regex syntax, since sometimes things are a little different from language to language, but it's worth a try!
A while ago I was doing some complex string parsing and found the following AMAZING webpage -- does what you described above, but has a LOT of flexibility, in switches you can turn on/off, etc. and even allows an "external" data source on which to test your patterns (and thus get your results)...
http://regexlib.com/RETester.aspx