MathExp
This math expression 

Example:
 var calc = new Calculator();
 calc.SetVar("a", 1);
 calc.SetVar("b", 2);
 Console.WriteLine("t0: " + calc.Calc("311"));
 Console.WriteLine("t1: " + calc.Calc("31+2-5*0"));
 Console.WriteLine("t2: " + calc.Calc("31+(2-5)*0"));
 Console.WriteLine("t3: " + calc.Calc("3.99*0.9*6.83*1.15"));
 Console.WriteLine("t4: " + calc.Calc("a+b"));