What constitutes the internal form of repentance (Tawbah Batina)?

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The internal form of repentance is that which is between the servant and his Lord. If the sin does not impose a right upon the person in the legal sense, such as an illicit kiss, seclusion with a forbidden person, drinking intoxicants, or lying, then repentance consists of regret and the firm intention never to repeat the act. It has been related that the Prophet, peace be upon him, said, "Regret is repentance." Some hold that sincere repentance (Tawbah Nasuh) comprises four elements: regret in the heart, seeking forgiveness with the tongue, resolving not to return to the sin, and avoiding bad company.