Both men genuinely regretted ----.

regret to do sth 后悔去做某事,还未做.
regret doing sth 后悔做过某事 做过了.
再问： 如果课文中是 both men were genuinely sorry that they had to leave. 那这个是，要离开，还是离开了？？
再答： 当然是要离开了。