Returns all the subdomains as an array, so ["dev", "www"] would be returned for "dev.www.rubyonrails.org". You can specify a different tld_length, such as 2 to catch ["www"] instead of ["www", "rubyonrails"] in "www.rubyonrails.co.uk".

Source Code

# File action_controller/request.rb, line 213
def subdomains(tld_length = 1)
  return [] unless named_host?(host)
  parts = host.split('.')
  parts[0..-(tld_length+2)]
end