Regret
Repentance
noun, verbal noun, の adjective
My behind regret is the regret from before, which is all regret. So this means regret, or repentance.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
本当に後悔はないんですね。
You really don't have any regrets, do you?
そんなことをしたら、自分が一生後悔の念に苛まれるだけだぞ！
If you do that, you'll only be tormented by regret for the rest of your life!
美人局にいとも簡単に引っ掛かってしまったことを後悔しています。
I regret how easily I got caught in her badger game.