[Feature Request] POSIX Username validator

[johnnybubonic]

• [X] I have looked at the documentation here first?
• [X] I have looked at the examples provided that may showcase my question here?

Package version eg. v9, v10:

v10

Issue, Question or Enhancement:

I'd like to suggest a "posixuser" validation.

You can find a regex pattern here.

I'll see if I can provide a PR if you think it'd be a good addition.

Code sample, to showcase or reproduce:

(N/A)

[mrusme]

I've come across this use case for the millionth time now over the past years and I think it would be useful to have this available by default.

Could any maintainer tell if this is something that would be accepted as a PR?