sub valid_domain {
  return $_[0] =~ m{
    \A
    (?:
      (?!\d+\.)
      [a-zA-Z0-9]+ (?: -+ [a-zA-Z0-9]+ )*
      \.
    )*
    (?!\d+\z)
    [a-zA-Z0-9]+ (?: -+ [a-zA-Z0-9]+ )*
  }x;