What are the allowed characters in a subdomain?

Question

What characters are you allowed to use in a subdomain?

Example: for someSub.example.com the someSub portion.

I know you can use le

Answer 1

Ironically, the other answers to this questions are correct answers to the question "what are the legal characters for a hostname or primary domain", while the answers to the question "can (hostname) subdomains have an underscore" are correct answers to this question.