Custom validator for Subdomains

subdomain_validator.rb

class SubdomainValidator < ActiveModel::EachValidator
  # http://matthewhutchinson.net/2010/10/27/rails-3-subdomain-validation-activemodeleachvalidator

  def validate_each(object, attribute, value)
    return unless value.present?
    reserved_names = %w(www ftp mail pop smtp admin ssl sftp)
    reserved_names = options[:reserved] if options[:reserved]
    if reserved_names.include?(value)
      object.errors[attribute] << 'cannot be a reserved name'
    end
    object.errors[attribute] << 'must have between 3 and 63 characters' unless (3..63) === value.length
    object.errors[attribute] << 'cannot start or end with a hyphen' unless value =~ /\A[^-].*[^-]\z/i
    object.errors[attribute] << 'must be alphanumeric (or hyphen)' unless value =~ /\A[a-z0-9\-]*\z/i
  end

end

Thanks ❤️
I created a javascript version of this with a small tweak.
https://gist.github.com/mohammadwali/af11bb30b7a687a8908c9496c49f4faa