Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
Regret in a dream indicates repentance and returning to God. Whoever sees himself regretting a sin, God will forgive him. Regretting a good deed indicates loss.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
Regret indicates salvation from perils. Whoever sees himself regretful will fall into a matter he regrets in wakefulness.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
Regret is interpreted as repentance. Whoever sees himself regretting selling something is attached to the world and abandoning the Hereafter.