code: func builtinRegExp(call FunctionCall) Value {
  	pattern := call.Argument(0)
  	flags := call.Argument(1)
  	if object := pattern._object(); object != nil {
  		if object.class == "RegExp" && flags.IsUndefined() {
  			return pattern
  		}
  	}
  	return toValue_object(call.runtime.newRegExp(pattern, flags))
  }
docs: // RegExp