discourse/lib/validators/reply_by_email_address_validator.rb

class ReplyByEmailAddressValidator
  def initialize(opts={})
    @opts = opts
  end

  def valid_value?(val)
    return true if val.blank?

    !!(val =~ /@/i) &&
    !!(val =~ /%{reply_key}/i) &&
    val.gsub(/\+?%{reply_key}/i, "") != SiteSetting.notification_email
  end

  def error_message
    I18n.t('site_settings.errors.invalid_reply_by_email_address')
  end
end