93 Copyright © 2013, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. Chapter 5 DOI: 10.4018/978-1-4666-3922-5.ch005 1. INTRODUCTION The formal definition of the semantics of visual languages has been the focus of many works, in order to extend the scope of such languages to more critical domains. For example, UML diagrams have been formalized using different formalisms, such as formal specification languages (PVS [Aredo, 1999; Ledang & Souquires, 2001], CSP [Ng & Butler, 2003], Z [Dupuy, 2000; France & Bruel, 2001; France, Bruel, Larrondo-Petrie, & Grant, 1997],...). Among these formalisms, graph transformation systems have known a fair success representing the visual languages seman- Najet Zoubeir Institut Supérieur d’Informatique, Tunisia Adel Khalfallah Institut Supérieur d’Informatique, Tunisia Samir Benahmed Faculté des Sciences de Tunis, Tunisia Expressing and Validating OCL Constraints using Graphs ABSTRACT The deinition of the semantics of visual languages, in particular Uniied Modeling Language (UML) diagrams, using graph formalism has known a wide success, since graphs it the multi-dimensional na- ture of this kind of language. However, constraints written in Object Constraint Language (OCL) and deined on these models are still not well integrated within this graph-based semantics. In this chapter, the authors propose an integrated semantics of OCL constraints within class diagrams, using graph transformation systems. Their contribution is divided into two parts. In the irst part, they introduce graph constraint patterns, as the translation into graphs of a subset of OCL expressions. These patterns are validated with experimental examples using the GROOVE toolset. In the second part, the authors deine the relation between OCL and UML models within their graph transformation system.