I'm trying to write a script where part of its functionality depends on if a user-provided domain is a zone apex (example.com) or not (www.example.com). What is a robust way to determine if I'm dealing with a zone apex or a subdomain? I'm hoping for a purely pattern-based approach but that seems tricky (example: xx.yy.co is not a root domain but xx.co.uk is).
Are there any tried and true approaches to determine if a zone is a root domain or not?
The Public Suffix List indicates lists of top level and second level domains under which one can register a domain name. If a name has exactly one more level beyond its matching entry on this list, then it's what you are looking for.
(Note that "subdomains" as you call them can be DNS zones in their own right and have independent nameservers from the parent zone. These can generally be detected by the presence of an SOA record for that fully qualified name, and nameservers for that name in the parent zone.)