What characters may I use when registering a domain name?
You can use a-z and 0-9 in all combinations, as well as hypens, but your domain name can not start or end with a hyphen. You can not use any other characters or a punctuation.

What is the minimum number of characters a domain name can have?
Most domain name extensions require a minimum of 2 characters for any domain name. Some countries have set higher minimum of characters for names registered with their extension.