Domain name Holder´s FAQ

A domain name can be up to 63 characters long and must have at least 3 characters. A domain name may only start with a letter or a number. Since January 1st 2011 village names may also be registered by any interested party. The only special character allowed by the DNS system is the hyphen (-) (apart from IDNs, see below), but this may not be at the beginning or the end of the domain name. Domain names containing other special characters cannot be registered unless they are contained on the list of special caracters accepted under .lu. We recommend that you only use lower case letters as the DNS system does not differentiate between lower case and upper case.
It is possible to register domain names containing accented characters, known as Internationalised Domain Names (IDNs), since February 1st 2010.

See section * Internationalized Domains Names (IDN)