Match valid subdomain string.

subdomain_matcher.rb

SUBDOMAIN_MATCHER = /\A([a-z][a-z\d]*(-[a-z\d]+)*|xn--[\-a-z\d]+)\z/i

[
  ['as a', nil],
  ['a-a', 0],
  ['todd2', 0],
  ['todd-2', 0],
  ['todd-admin', 0],
  ['_todd-admin', nil],
  ['-todd-admin', nil],
  ['2todd-2', nil],
  ['todd_admin', nil],
  ['a_a', nil]
].map do |data|
  subdomain, expected = data

  (subdomain =~ SUBDOMAIN_MATCHER) == expected
end