It really depends on the restrictions put in place by the registry of a specific domain extension.

The following are the current restrictions in place for gTLD domains.

• A domain name can be up to 63 characters long plus a dot plus the characters used to identify the top-level domain (i.e "com", "info", "biz", etc.
• Valid characters in a domain name include letters, numbers and hyphens "-". The domain name must start and end with a letter or number.

For country code and other domain extensions, you should check with the registry for the specific country code or domain extension.