I googled "unit test mvc model validation" and this was the first result
TDD for ASP.NET MVC Part 4: Unit Testing View Model Validation -- Visual Studio Magazine[
^]
If you google yourself you'll find others. In general though you should only unit test your own code, the model validation is Microsoft's code and you should simply assume they have tested it themselves and that it works. Saying that, however, as you have regex validation there probably is still value in testing that your regex is working as expected so unit testing in this case is probably ok to do.