What is the ruling regarding someone who repented from abandoning prayer and now prays regularly, but does not regret abandoning it in the past? Is his repentance accepted?

Regret is a fundamental pillar for the soundness of repentance. So, whoever does not regret abandoning prayer is not truly repentant. Whoever does not feel regret for abandoning prayer must strive to achieve it for his repentance to be valid, and he must know that he has committed a great and serious sin. He must recall the enormity of the sin of abandoning prayer to feel what punishment awaits him.