module ActionDispatch::Http::URL

def extract_subdomain(host, tld_length)

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

Returns the subdomains of a host as a String given the domain level. 
def extract_subdomain(host, tld_length)
  extract_subdomains(host, tld_length).join(".")
end