Page "Domain Name System" Paragraph 19

* DNS names may technically consist of any character representable in an octet.

However, the allowed formulation of domain names in the DNS root zone, and most other sub domains, uses a preferred format and character set.

The characters allowed in a label are a subset of the ASCII character set, and includes the characters a through z, A through Z, digits 0 through 9, and the hyphen.

This rule is known as the LDH rule ( letters, digits, hyphen ).

Domain names are interpreted in case-independent manner.

Labels may not start or end with a hyphen.