Achieving a quality software system requires UML designers to have a good understanding of both design patterns and antipatterns. Unfortunately, UML models for real systems tend to be huge and hard to manage, especially for models automatically generated from source code. Thus, it would be advisable to have tools to automatically identify particular instances of patterns. So, a formal language to express them is needed. However, a textual formalization of such a language is barely usable by UML practitioners. In this paper we propose a visual notation obtained by adding to UML as few graphical elements as possible in order to express both patterns and antipatterns (with the needed formality). This approach has low cognitive load so is easily usable by practitioners but is still rigorous enough for implementation. This notation will be exploited by a GUI front-end for a prototypical tool (that we have recently developed) which is able to discover (anti)patterns in models.