Question
can i use the herbicide "POST" to kill the grass and not kill the lilies?

Answer
I am going to presume that you are talking about POAST , a brand name that contains Sethoxydim as the active ingredient.
Click Here to view a label (Hi-Yield) of a product that contains Sethoxydim. Scroll to page 15 and you will see that the product is listed as safe on two 'lilies'.
One is the "Lily Of the Valley" (Family: Asparagaceae ).
The other is "Lily of the Nile" (Family: Amaryllidaceae .
You may wonder why all the details - and you would be right!

Anything that kills any plant can be rightfully called a plant killer.
All weed killers are plant killers.
The secret to success is in the dose. If you exceed the application rate, you can kill your desirable plant!
And to complicate things further, the dose is different for different plants - right down to the variety!
So, to determine whether the chemical you have - in mind or in hand - is safe for your plant, look for it on the label in the "safe for use on" section. If you do not see the specific plant, look for one in the same family. Bear in mind that the farther away you get, the chemical gets more and more 'unsafe'.
I hope that this helps.