RFC compliant. Disallows '.' (period) to be at the front or end, or occurr more than once consecutively. It does allow for optional infinite number of sub-domains. However, the total length of the URI has not been enforced: each label must be between 1 and 63 characters long, while the total length of the domain name may not exceed 255 characters.
Note that this RegEx does not check the validity of the protocol.