domain names must:
a) be at least 2 characters long;
b) contain only letters (a-z), numbers (0-9) and hyphens (-), or a combination of these;
c) start and end with a number or a letter, not a hyphen; and
d) not contain hyphens in the third and fourth position (e.g. ab--cd.com.au).
There is no distinction between upper case and lower case letters in a domain name.