jQuery.validator.addMethod("valid_username", function(value, element) {
        return this.optional(element) || value.search(/^[a-z0-9]{6,16}$/gi) != -1;
      }, lang.js_login_invalid_username);

      var sign_up_form_validate = $('#sign_up_form').validate({
        rules: {
          username: {
            required: true,
            valid_username: true,
            remote: {
              url: site_url + "login/check_username_exist",
              type: "post",
              data: {
                username: function(){
                  var username = $("#username").val();
                  return username;
                }
              }
            }
          },
          firstname: 'required',
          lastname: 'required',
          email: {
            required: true,
            email: true,
            remote: {
              url: site_url + "login/check_email",
              type: "post",
              data: {
                email: function(){
                  var email = $("#email").val();
                  return email;
                }
              }
            }
          },
          password: {
            required: true
          },
          password_again: {
            required: true,
            equalTo: "#password"
          }
        },
        messages: {
          username: {
            required: lang.js_login_required_username,
            remote: lang.js_login_username_already_exist
          },
          firstname: lang.js_login_required_firstname,
          lastname: lang.js_login_required_lastname,
          email: {
            required: lang.js_login_required_email,
            email: lang.js_login_valid_email,
            remote: lang.js_login_valid_email
          },
          password: {
            required: lang.js_login_require_password
          },
          password_again: {
            required: lang.js_login_require_password_again,
            equalTo: lang.js_login_password_no_equal
          }
        }
      });