What is the expiation for someone who cut off a dog's tail, and now regrets this action, not having known of its prohibition?

Islam is a religion of mercy that encourages kindness to animals and forbids torturing them. Regretting what you did is repentance, and he who repents of a sin is like one who has no sin. You are not required to offer any specific expiation other than sincere repentance, unless the dog was owned by someone and permission was granted for its ownership. In that case, some scholars hold that there is compensation for the damage (i.e., its value or the depreciated amount for its owner). The preponderant opinion is that the value of the dog is not obligatory if it is destroyed, because it has no legal compensation.