Valid domain names
From Bravenet Wiki
Domain names are required to meet certain strict naming requirements. Although there are efforts underway to allow domain names with non-latin alphabets (for example, Greek, Chinese, Hebrew, etc) currently most systems do not allow such names.
Domain Name Limitations
Domain names must only use the letters a to z, the numbers 0 to 9, and the hyphen (-) character. If the hyphen character is used in a domain name, it cannot be the first character in the name. For example, -example.com would not be allowed, but ex-ample.com would be.
Each part of a domain name must follow these restrictions - a domain name x.y.z.co.uk would have to ensure that the x, y, z, co, and uk portions of the domain name all follow these rules. This means that subdomain names need to follow the same rules as domains. In addition, the entire domain name, including the TLD, can only be a maximum of 67 characters. This includes the periods between each label - www.example.com is 15 characters long.
Domain names are subject to restrictions due to trademark and copyright laws. Most of the time, the owner of a registered trademark is able to contest the ownership of a domain name that mimics the registered trademark. This can be a very murky area, but in general it is highly recommended that you not register domains that infringe on registered trademarks unless you are the holder of that trademark.
Some top level domains, especially those managed by individual countries, may have additional restrictions. For example, .biz domains specify that they should be used for business or commercial use only, and .name domains should represent your name, or a name that you are referenced by.
In general, each part of a domain should be as self-explanatory as possible. A short, informative, easy-to-remember domain name will go far in ensuring that your website is memorable. In addition, it is often a good idea to break up logical portions of your website onto subdomains of a main domain. A great example of this is Google - they have their mail service on mail.google.com, their mapping service on maps.google.com, and so on.
Also, choosing an appropriate top level domain (TLD) can make your domain name much more memorable. Although many people want to get a .com domain name, a well-thought out .org or .net name can be more memorable, and easier to obtain, than a .com name. For example, which is easier to remember - ex4mple-4432.com or example.biz? There is so much competition for .com names that most of the "good" names are already taken. Through Bravenet you can register for most of the unrestricted TLDs (see the article on TLDs or registering a domain for specifics); in addition, you can use domain names registered elsewhere for your Bravenet websites, or transfer your domain to Bravenet.