The problem is that QLineEdit feeds QCompleter with its text without taking input mask into consideration. In your situation, typing for example "A" passes "A____-_____-_____-_____" to the completer, which naturally matches none of the serials. What you could try is to use QCompleter::UnfilteredPopupCompletion. I know it doesn't work very well either but at least it shows something.

PS. You could send a suggestion to the Trolls via task-tracker. At least I think that it would be a nice option to have that no useless input mask characters would be passed to completer. Of course, it should only pass everything up till the first empty character in case one enters one character here and another there, like "A____-B____-_____-_____" would pass "A".