What is the ruling on the marriage of a man and a woman who had a spousal relationship before completing their legal marriage? And what is the expiation for what they did?

Adultery is a grave sin that expels its perpetrator from faith and exposes them to torment. Its punishment in this world is stoning for the married and lashing for the unmarried. Whoever is afflicted by it must repent, increase good deeds, and seek to be covered by Allah's concealment. It is impermissible to marry an adulterer or an adulteress unless they have repented. If a marriage contract was concluded before their repentance, it is invalid, and they must repent and remarry.