The paper deals with the problem of output regulation for a class of hybrid linear systems and exosystems whose state jumps periodically according to a known clock. In this framework the main contribution of the paper is to present a general method for the systematic design of robust internal model-based regulators by extending results that are known for continuous-time systems. The internal model design procedure relies upon a notion of visibility of the so-called "hybrid steady-state generator." The general theory is applied to the case of robust tracking of spline-based reference trajectories by showing how the latter can be thought of as being generated by hybrid linear exosystems. (C) 2016 Elsevier Ltd. All rights reserved.