What rules are there that domain names must constrain to?

[harleyx]

I know of the following:

Only contains alphanumeric characters and dashes.
No dashes at the beginning or end of the name.
63 character maximum (on most common extensions)

Are there any other constraints to the structure of a domain name I'm missing?

 

[DomainingCom]

Yes, some TLD does not allow domain names having less than 3 characters.
Exemple: .mobi