A method includes accessing a validator routine having an input string and one or more return points, each return point returning a return value having two possible values; finding the return points in the validator routine; for each of the return points, performing a backwards traversal from a return point through a code section and determining constraints on the input string based at least on one or both of the two possible return values for the return point; using the determined constraints for the input string, determining whether all of the return values returned from the one or more return points meet validation constraints; and outputting one or more indications of whether all of the returned values returned from the return points meet the validation constraints for the one or both of the two possible return values. Apparatus and computer program products are also disclosed.