Update (2014-07-15): Never mind me!
I recently came across another tool that does just what I intended to do and not only that, it does it quite a lot better. I think that if you like/use this tool or if you’re looking for something like it, check out Regex Hero. Great tool, quick and easy to use with lots of nice features. Basically it’s the exact same thing as this here since it runs in the browser using Silverlight but it’s a lot more polished. Free to use although they’ve got paid versions with advanced features like explaining your regex in plain English, intellisense, recommendations or a desktop version.
Regex Hero
Update (2013-12-01):
Found that there was a problem in selecting from resulting text, also found it quite hard to use when so incredibly small so here is a larger version of regex validator.
I think it’s hard to find a good online regex validator/tool, especially for .Net usage so I thought I’d write one myself so here it is, in Silverlight. I’ll post the code sometime in the future aswell.