How to add voltage thresholds with hysteresis to the input of a voltage regulator?
As you have an enable pin on your regulator, the easiest way is to use something like a comparator to turn it on or off. 

All you will have to do it set up a reference voltage and when your goes below the reference, your regulator sill switch off. Once it is above the reference again, it can come back on. 

EDIT After the comment from @Transistor, I will add that if you use 2 comparators, you can give yourself an upper and lower threshold. This design is known as a Window Comparator. You could also consider using a schmitt trigger.