About the only thing you can rely on is that most (all?) registries
enforce those character restrictions in domains registered with them.
I just checked, none of the domain I register via Tucows will accept a
name that starts with an underscore, although of course there are lots
of other TLDs and lots of delegation points below the TLD.
So anyway, as I said the first time this came up, this is a
contractual issue between a domain and its superdomains, and is merely
one example of the general rule that if your superdomain is
sufficiently hostile or incompetent, you lose.
Since that problem affects every use of the DNS, I don't see any point
in us documenting it here.
NOTE WELL: This list operates according to