In regular expressions , the valid operators are :
() [ Parantheses ] , '.' [ Concatenation ] , '+' [ Union or choice ] , '*" [ Kleene's Closure ] , '+' [Positive Closure]
Hence a* - b* is not a valid regular expression.
However the minus operator can be used to denote difference of any two languages i.e. L1 - L2 is valid suggesting that the strings which are in L1 but not in L2..
Hence L1 - L2 is valid irrespective of language class of L1 and L2..