Hi,

Thanks for the PR!
Correct me if I'm wrong, but I don't think domains should be allowed to have underscores, only subdomains, right?
As for the coverage issue, you're never triggering the error case, you'll need another case with that new option and something such as _abc.{example}.com

@Marsup as per the RFC the underscore could appear anywhere. In practice, I'm not aware of second-level domains that use it, but it isn't forbidden (at least not directly in the spec but each TLD can impose additional restrictions):

The DNS itself places only one restriction on the particular labels
that can be used to identify resource records. That one restriction
relates to the length of the label and the full name. The length of
any one label is limited to between 1 and 63 octets. A full domain
name is limited to 255 octets (including the separators).

Trying to register a domain with an underscore doesn't seem to work in any of the registrar I've tried. Some even mention that underscore is not an allowed character.

Well, that's true, but that doesn't mean much:

1. The RFC document defining the syntax doesn't disallow underscores, and therefore - there are DNSes operating with underscore present. The letsencrypt DNS is the best example I could think of.
2. Most (if not all) DNS servers allow for subdomains with underscore to be registered.

As for the coverage issue, you're never triggering the error case, you'll need another case with that new option and something such as _abc.{example}.com

I tried that, and it didn't work for me. Maybe somebody else could play around and try to figure it out.

Weird, adding ['_abc.{example}.com', false, { allowUnderscore: true }] does it for me.

Weird, adding ['_abc.{example}.com', false, { allowUnderscore: true }] does it for me.

You're right, It passed. Thanks!

FYI I haven't forgotten about this PR, but since I know you're probably expecting it in joi afterwards, I need to figure out (which is mostly done) why the new address adds so much to the browser bundle, and how to come back to a better place, but this PR is valid as is.