http://www.hscripts.com/tutorials/web/domain-characters.php
Domain Name:
A Domain Name can have alphabets, numbers and hyphen (-).
The following are the main criteria or characteristics that a domain name can have.
a) Alphabets are accepted (e.g: withfriendship).
b) Numbers are accepted (e.g: 12343121).
c) Combination of alphabets and numbers (alphanumeric) are accepted (e.g: 1d2f2g).
d) Only non alphanumeric character that is accepted is hyphen (-) (e.g: asdd-asdd). The name can neither start nor end with the character "-", although the character "-" is allowed inside the name.
e) Maximum length of domain name can not be more than 64 characters long (including extensions like .com).
f) A valid domain extension (e.g: .com, .net, .org )
The name that does not follow the above criteria will not be accepted for
registration.
Also domain names are case in-sensitive.
They can not be differentiated based on lower or upper case letters.
i.e HIOXINDIA.com == hioxindia.com, both are same.