Always regret evil done
and good not done.
Never regret good done
and evil not done.
In Buddhism, regret is not guilt. Guilt is an ego-driven emotion and as such is always non-virtuous, but regret can be either virtuous or non-virtuous. It is virtuous when we have a strong regret for the negative things we have done in the past, consciously or unconsciously, acting under the power of attachment, anger, jealousy, and the like.
To see that the harmful actions of body, speech and mind are negative and should be avoided, and to feel regret that we have done them, is positive in that it leaves a definite imprint on the mindstreams that will help us to avoid such actions in the future.
TDEditor: Non-virtuous regret refers to regret for having done the positive, which is beneficial for oneself and/or others, also via body, speech and mind. Such regret also arise from deluded defilements. Regret for having been virtuous is negative in that it leaves a definite imprint on the mindstreams to avoid being virtuous in the future.
Geshe Tashi Tsering