There are lots of way to accidentally lose a leading zero along the way. For a quick and realistic example make a csv file the looks like:

   01234,Hello

import it into Excel, edit something and export it as csv again. Unless you pay careful attention to what you're doing, you've just lost your leading zero.

Agreed - if you're going to validate anything at all, define a new type and don't hide the constraints checking within a secret unit testing lair. I hope I would be agnostic about the code here though, there is no point rewriting poorly conceived stringly typed code - that's a symptom that such efforts would be part of a losing battle

The code was for handling zipcodes from a CSV file so we don't really know where the data came from.