In the frontend, flexible intervals are not validated. For instance, a user can create a flexible interval specified as "asdf,123".
While we are at it, it would also be nice to support flexible intervals with a single day. Currently, flexible intervals are expected to have day1-day2 in their specification, like so: "1-5,10:00-18:00". If, for instance, only Monday is desired, we now wish to allow "1,10:00-18:00".