﴿ وَإِنَّهُۥ لَحَسْرَةٌ عَلَى ٱلْكَٰفِرِينَ ﴾

“yet, behold, this [rejection] will indeed become a source of bitter regret for all who deny the truth [of God's revelation] –”

(And lo! It) i.e. the Qur'an (is indeed an anguish) a remorse (for the disbelievers) on the Day of Judgement.