What is the Sharia ruling on a divorced woman who did not adhere to the waiting period (iddah), and is it obligatory for her to begin it now?

The iddah (definition) (waiting period) of a divorced woman begins from the moment the divorce occurs. Its meaning is a specific period prescribed by Islamic law to prevent a divorced woman with whom consummation has occurred, or a widow, from remarrying. For the divorced woman, observing the iddah means refraining from marrying another man during this period, and, according to the more correct view, the obligation to reside in the marital home during it. If she marries during the iddah, the marriage is invalid. If she leaves the residence, she has sinned and must repent. Nothing else is required of her besides refraining from marriage and remaining in the marital home.