Regret over a past sin.

The Shaykh : The second: Regret over what? Regret—no—over what has passed, over the sin. To regret means to feel remorse and to say in one's heart, "I wish I had not done it," because some people may commit a sin but do not regret it, meaning that to them doing it and not doing it are the same. But one regrets, feels remorse, laments, and says in one's heart, "I wish I had not done this." This is the regret.
It was difficult for some scholars: How can regret be a condition when regret is a psychological reaction that cannot be demanded? It is said: The meaning of regret is that the effect of committing the sin appears on the person, meaning that he feels remorse and says, "I wish I had not done it." Good.