SQL & PL/SQL :: Multiple Report Parameter Values

For the below query:

SELECT id, pob,exp
FROM emp
where exp= (:exp)or @exp is null
O/P:
id pob exp
_____________________
1 CT 2
2 NJ 3
3 NY 2

It takes only one value of :exp but, I would like to give multiple values separated by ','. My problem is the parameter can be either single value or Blank or multiple values. My code do work for single value and Blank, Now how to implement multiple values .