When you do something in a way that expresses regret, you’re doing it ruefully. If someone asks you about your Saturday night and you smile ruefully in response . . . well . . . you probably made some poor choices.
The origin of the word ruefully is a Germanic word that means “repentance.” But over time the word has adopted a slightly humorous tone, so you can go ahead and shake your head ruefully at your friend who has just put his boot on the wrong foot — you feel a little bit sorry for him at the moment. You might grin ruefully when asked about how many pairs of shoes you bought on your trip.