code: function(inputValue, ruleValue, attribute, callback) {
      var _this = this;
      this._setValidatingData(attribute, inputValue, ruleValue);
      if (typeof callback === 'function') {
        this.callback = callback;
        var handleResponse = function(passes, message) {
          _this.response(passes, message);
        };
  
        if (this.async) {
          return this._apply(inputValue, ruleValue, attribute, handleResponse);
        } else {
          return handleResponse(this._apply(inputValue, ruleValue, attribute));
        }
      }
      return this._apply(inputValue, ruleValue, attribute);
    }
docs: Validate rule
  
  @param  {mixed} inputValue
  @param  {mixed} ruleValue
  @param  {string} attribute
  @param  {function} callback
  @return {boolean|undefined}