We present three dynamic error feedback controllers for robust output regulation of regular linear systems. These controllers are i) a minimal order robust controller for exponentially stable systems, ii) an observer-based robust controller, and iii) a new internal model based robust controller structure. In addition, we present two controllers that are by construction robust with respect to predefined classes of perturbations. The results are illustrated with an example where we study robust output tracking of a sinusoidal reference signal for a 2-D heat equation with boundary control and observation.