What characters can I use in a domain name?

A domain name must meet the following technical conditions:

• it must contain at least 2 characters from the set 'a'..'z', digits '0'..'9', and the hyphen '-'
• it must be at most 63 characters long
• it cannot start or end with a hyphen '-'
• it cannot have a hyphen '-' in both the 3rd and 4th positions at the same time