Except for the Madrid event, you can leave and re-enter by keeping your ticket and registering it when you leave.

In Madrid, you can enter and leave the pavilions to the central space of Ifema, but once you leave the Ifema grounds you cannot re-enter. There is a VIP ticket that does give you the right to leave the grounds and re-enter, but this is not possible with the other tickets (General and Anti-crisis).