EBNF for domain names (copied from https://www.ietf.org/rfc/rfc1035.txt)

grammar.txt

<domain> ::= <subdomain> | " "

<subdomain> ::= <label> | <subdomain> "." <label>

<label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]

<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>

<let-dig-hyp> ::= <let-dig> | "-"

<let-dig> ::= <letter> | <digit>

<letter> ::= any one of the 52 alphabetic characters A through Z in
upper case and a through z in lower case

<digit> ::= any one of the ten digits 0 through 9

or better yet, since it's producing strings of the form: <label> ("." <label>)* just write it as:

<subdomain> ::= <label> {"." <label>}