God saw their actions, that they had turned from their evil ways. Jonah 3:10
When confronted by God through Jonah, the Ninevites repented of their evil. Note that the change was visible. Repentance, by definition, is a change of direction or turning around. The Ninevites turned from their way.
When faced with my sin, I feel regret. I am sorry for what happened but it may stop there. Regret is merely a pathway to repentance and I must push forward to change.
If I have hurt you and tell you I’m sorry, you can judge my sincerity on what I do to ensure it doesn’t happen again. The same is true in my relationship with God. He knows my heart but I may not. Coming to Him in sorrow or regret is not the same as repentance.
Regret could and should lead to repentance, but am I feeling sorrow merely for the consequences of my transgression? Or is my sorrow in hurting God? The former is rarely motivation to ensure lasting change.
God knows my heart. And He also sees my actions. Regret is the beginning of repentance, whether towards you or to God, and gratefully can be released after it has achieved its desired work in restoring me to a right path.
And now I rejoice, not because you were made sorrowful, but because your sorrow led you to repentance. 1 Corinthians 7:9