Conditional text in a positive lookbehind grep sequence

I'm trying to create a grep string that will subscript a symbol but only after specific words and not all instances of the symbol. 

In the example below I want the % subscripted when used after Melon and Apple but no other use of % should be subscripted.

I cant figure out what I'm doing wrong?

(?<=[Grape|Orange|Apple|Bananna|Avocado|Melon|Pineapple])%