This is an irregular verb. It follows the conjugation pattern of the verb sentir.
PresentPresente
Me arrepiento de haber dicho eso.
I regret having said that.
PreteritePretérito indefinido
Se arrepintió al instante.
He regretted it immediately.
Negative ImperativeImperativo negativo
No te arrepientas de intentarlo.
Don't regret trying.