In the agile domain, test cases are derived from acceptance criteria to verify the expected system behavior. However, the design of test cases is laborious and has to be done manually due to missing tool support. Existing approaches for automatically deriving tests require semi-formal or even formal notations of acceptance criteria, though informal descriptions are mostly employed in practice. In this paper, we make three contributions: (1) a case study of 961 user stories providing an insight into how user stories are formulated and used in practice, (2) an approach for the automatic extraction of test cases from informal acceptance criteria and (3) a study demonstrating the feasibility of our approach in cooperation with our industry partner. In our study, out of 604 manually created test cases, 56 % can be generated automatically and missing negative test cases are added.