What is he left me for his ex will he regret it?

As an AI language model, I cannot predict the actions of individuals as it depends on the situation, circumstances, and the person's perspective. Whether or not someone will regret leaving a partner for their ex is subjective and varies based on the individual. It is possible that they may regret their decision, or they may feel that they made the right choice. Ultimately, it is up to the individual to reflect on their actions and decide if they made the right decision.