For proving regular language as unambigious you must know that first of all regular language are either left linear or right linear...for any string of this regular language if you try to draw parse tree you will always get unique derivation tree...this is due to the condition of type 3 grammar that limits you on going either left side or right side..you never can go on both sides nor we can have more than 1 variable so anyhow you cant derive more than 1 parse tree for any string in regular language..! (Regular are generated by type 3 grammars..!)
