Custom primitive types for verifying a specific model of something

[mtmr0x]

In discussion with my friend Hugo Bessa, we spoke about how could I make sure a ISO date would be always a ISO date and creating other types might not be a great idea but what about my primitive type be custom somehow?

Examples

Custom primitive number type

I know it's a number, but I can create rules for this number don't be greater than x or non-zero values

Custom primitive string type

It must be a string, but is a specific type of string, as a only numeric string or not having spaces, validated by a regex, for example.