The Kaukṛtya Caitasika Dharma

The mental factor of regret involves remorse, repentance, condemnation, and aversion toward one's past actions. It sometimes belongs to wholesome mental factors and sometimes to unwholesome ones. The manas (mind-root) of ordinary beings and those in the three virtuous stages experiences regret that is sometimes wholesome and sometimes unwholesome. Even saints who have eradicated afflictions and purified their minds possess regret in their manas, but it is always wholesome. However, if someone regrets having performed wholesome deeds, that regret constitutes an unwholesome factor. Bodhisattvas above the eighth ground, who have exhausted all afflictions and habitual tendencies, have no regret in their manas, because they no longer commit wrongdoings. The Buddha's manas has no mental factor of regret whatsoever, due to his supreme wisdom. If the manas of hell beings regrets their evil karma, this is wholesome; the evil karma is immediately destroyed, and they will promptly emerge from hell. The consciousness of hell beings, being preoccupied with suffering, has no mind to regret. Even if the consciousness could regret, it could not eliminate evil karma and cause emergence from hell, since the consciousness is not the master.