Validate domain with regex but not subdomain - regex

Question

I couldn't find anywhere a regex that could validate a domain but not accepting subdomains.
I found a lot of rules that validates domains but unfortunately all of them also validates subdomains.
Anyone have tips on this?
I have this regex that is almost what I need:
/(?!www\.)(?=^.{5,254}$)(^(?:(?!\d+\.)[a-z0-9\-]{1,63}\.){1,2}(?:[a-z]{2,})$)/
If I use a subdomain like test.domain.com.br, it validates good (rejecting it), but test.domain.com don't.

Answer 1

I couldn't find anywhere a regex that could validate a domain but not accepting subdomains.
Because no regex can do that for you (and anyone pretending the opposite just doesn't understand the DNS).
Which is exactly why you found out that:
a lot of rules that validates domains but unfortunately all of them also validates subdomains.
Because a "subdomain" is just a domain seen differently (or you can say that any domain is also a subdomain of another domain, except for root and TLD). This is all because the DNS is a tree.
You can use the definition given in https://www.rfc-editor.org/rfc/rfc8499:
Subdomain: "A domain is a subdomain of another domain if it is
contained within that domain. This relationship can be tested by
seeing if the subdomain's name ends with the containing domain's
name." (Quoted from [RFC1034], Section 3.1) For example, in the
host name "nnn.mmm.example.com", both "mmm.example.com" and
"nnn.mmm.example.com" are subdomains of "example.com". Note that
the comparisons here are done on whole labels; that is,
"ooo.example.com" is not a subdomain of "oo.example.com".
You can not find administrative boundaries given an hostname by just looking at it. You need either to do DNS live queries to find the delegation points OR you need to use something like the Public Suffix List maintained by Mozilla. Both cases have drawbacks that can be or not a problem depending on your use case.
If you are not convinced, here is some list of valid hostnames (you can use them in an URL and it will work), and try to find out how a regex could have helped you by being right in all cases:
dk
www.sante.gouv.fr
www.com.com
www.nominet.co.uk
www.uk.com
www.walton.k12.fl.us
lagazettedesancetres.blogspot.fr
www.al.ma.leg.br
ab.m.wikibooks.nom.nu
1512f1.станок.спб.рус
You can obviously find shortcuts where a regex will still be wrong but good enough, if you restrict the cases you need to act on. Otherwise, if you need to stay generic and potentially work in any TLD, then, sorry, no regex will solve your problem.
Also your regex is wrong in multiple other cases. For example it won't handle IDN TLDs, that do exist, as they will be like xn--something in ASCII form which won't be accepted by [a-z]{2,}
BTW, useful terminologies I suggest using which may often be clearer than domain/subdomain, as taken from https://url.spec.whatwg.org/#host-miscellaneous
"A host’s public suffix is the portion of a host which is included on the Public Suffix List."
"A host’s registrable domain is a domain formed by the most specific public suffix, along with the domain label immediately preceding it, if any."
I think what you are searching is the "registrable domain" part of any given string (and as you can see from the algorithm given at above URL, you can't do that without finding first the public suffix, which you can't do without using an external resource, the information is NOT self contained in the string).