What is the ruling on swearing an oath by expiation (kaffarah) to abandon something, then doing it inadvertently or intentionally? And does regret result from that?

The jurists differed concerning one who swore not to do something then did it out of forgetfulness: does he break his oath or not? The majority held that he does not break his oath except in matters of divorce and emancipation. The Shāfi‘īs, according to the more prominent view, held that he does not break his oath at all. Whereas the Hanafīs and Mālikīs held that he breaks his oath by violating it, even out of forgetfulness. The more cautious approach is to adopt the view of breaking the oath and undertake what results from it. As for intentionally doing what one swore not to do, that is undoubtedly a broken oath and entails expiation for an oath.