If you accidentally delete too much text while typing, simply shake your phone side-to-side to undo.

Whenever you make a typing mistake or accidentally delete a string of text on your iPhone, iPad, or iPod Touch, there is a simple way to Undo (similar to Edit/Undo on your computer). Simply shake your device side-to-side and a little box will appear asking you if you would like to Undo. Tap the button to Undo your last action. If you want to Redo your last action, shake your device again and you will be given an option to Undo again or Redo.