domain.rb

# first we will give Domains a little help

class Domain < String
  def subdomain( rank = 1 ) ; self.split('.')[0..(rank-1)].join('.') ; end
  def level( rank ) ; self.split('.')[(rank*-1)..-1].join('.') ; end
  def top_level ; level(1) ; end
end

# now we can treat them like strings but also access domain components naturally

d = Domain.new("store.apple.co.uk")
d == "store.apple.co.uk"              # => true
d =~ /^store/                         # => 0
d.subdomain                           # => "store"
d.subdomain(2)                        # => "store.apple"
d.top_level                           # => "uk"
d.level(2)                            # => "co.uk"
d.level(3)                            # => "apple.co.uk"

# within request mappings, we can pass lambda constraints:

:domain => lambda { |d| d.level(2) == "co.uk" }