start = EXPR

   EXPR = EXPR_M ( [-+] EXPR_M )*

   EXPR_M = EXPR_H ( [*%/] EXPR_H )*

   EXPR_H = V

   EXPR_H = '+' V

   EXPR_H = '-' V

   EXPR_H = '(' EXPR ')' 

https://en.wikipedia.org/wiki/Recursive_descent_parser