Regular expressions are quite possibly the least enjoyable thing about programming, mostly because I can't read them they're terrible.

Unit Tests allows you to make big changes to code quickly. You know it works now because you've run the tests when you make the changes you need to make, you need to get the tests working again. This saves hours.

This post not for how to write unit testing, it focuses on how to know the fail reason of your unit testing.

Data annotation is not enough for you and you looking for another framework to building validation rules.