Options to incorporate the undo function direcly into the mobile keyboard.
Keyboard
Tap the Undo Button integrated in the keyboard in-between the shift-key and z-key.
Mechanic: Touch
One-finger press, lift
(implimented with release of Apple IOS 9, in Septemper 2016: Apple replaced Cut/Copy functions on the keyboard with an Undo/Redo)
Tap the Undo icon, located on the left side of the row over the keypad.
Mechanic: Touch
One-finger press, lift
Hold the Z-key until the pop-up menu apears and chose the Undo icon.
Mechanic: Long press
One-finger press, wait, lift
Hold the Z-key for a while, till the undo command is activated and the device judders.
Mechanic: Hold
One-finger press,wait for a moment, lift