Expr -> Expr '+' Factor
Expr -> Expr '-' Factor
Expr -> Factor

Factor -> Factor '*' Exponent
Factor -> Factor '/' Exponent
Factor -> Exponent

Exponent -> Term '^' Exponent
Exponent -> Term

Term -> '(' Expr ')'
Term -> %f

%f -> /[-]?(0|[1-9][0-9]*)(\.[0-9]+)?([eE][+-]?[0-9]+)?/