A regular matrix pencil sE − A and its rank one perturbations are considered. We determine the sets in C ∪ {∞} which are the eigenvalues of the perturbed pencil. We show that the largest Jordan chains at each eigenvalue of sE − A may disappear and the sum of the length of all destroyed Jordan chains is the number of eigenvalues (counted with multiplicities) which can be placed arbitrarily in C ∪ {∞}. We prove sharp upper and lower bounds of the change of the algebraic and geometric multiplicity of an eigenvalue under rank one perturbations. Finally we apply our results to a pole placement problem for a single-input differential algebraic equation with feedback.Keywords: regular matrix pencils, rank one perturbations, spectral perturbation theory MSC 2010: 15A22, 15A18, 47A55 S(sE − A)T = s I r 0 0 N − J 0 0 I n−r , r ∈ {0, 1, . . . , n}