These words are not antonyms. They are not opposites.
* Regret is a feeling of sadness or disappointment about something that has happened or something that you have done.
* Grief is a deep sorrow, especially that caused by someone's death.
While they both involve negative emotions, they are different in their meaning and context.
For example:
* You might feel regret for not studying harder for an exam.
* You might feel grief after the loss of a loved one.
Let me know if you have any other questions!