We present an operational model for describing random reactive systems. Some models have already been proposed for this purpose, but they generally aim at performing global reasoning on systems, such as stochastic analysis, or formal proofs. Our goal is somehow less ambitious, since we are rather interested in executing such models, for testing or prototyping. But on the other hand, the proposed model is not restricted by decidability issues. Therefore it can be more expressive: in particular, our model is not restricted to finite-state descriptions.

The proposed model is rather general: systems are described as implicit state/transition machines, possibly infinite, where probabilities are expressed by means of relative weights. The model itself is more an abstract machine than a programming language. The idea is then to propose highlevel, user-friendly languages that can be compiled into the model. We present such a language, based on regular expressions, together with its translation into the model.