I'd say that it should be an warning, or a do-nothing. The + modifies the previous assertation. In this case, the previous assertation is (?#...), which asserts nothing about the stream. Asserting nothing a bunch of times should have the same effect as not asserting nothing at all, or asserting nothing once -- no effect. Of course, asserting nothing more then once probably isn't what you meant, but there's no way of telling what you did mean, so we should warn.