module ActionDispatch::Http::URL

extract_subdomains('dev.www.example.co.uk', 2) # => ["dev", "www"]
# Second-level domain example
extract_subdomains('www.example.com', 1) # => ["www"]
# Top-level domain example

Returns the subdomains of a host as an Array given the domain level.

def extract_subdomains(host, tld_length)
  if named_host?(host)
    extract_subdomains_from(host, tld_length)
  else
    []
  end
end