undo back button

I was almost finished typing out a thoughtful, somewhat long paragraph when I accidentally tapped the back button on the left-hand side of my iPhone 6's keyboard in landscape mode, and the entire paragraph was deleted. I then tapped Undo and it just kept deleting. If that back button is going to exist in such a precarious spot, there must be a redo button. I'm not finding such a feature and am hoping to get someone's insight as to how I can get my paragraph back. Thanks much!