Infix to Postfix

main.cs

using System;
using System.Collections.Generic;
					
public class Program
{
	public static void Main()
	{
		 //Console.Write("Infix Formule: ");
		 //string infix = Console.ReadLine();
		 string infix = "4.5+a5+.1215  +   1";
         String postfix = InfixToPostfix(infix);
         Console.WriteLine("InFix   :\t" + infix);
         Console.WriteLine("PostFix :\t" + postfix);
		
		
		infix = "(5.0  /   .9)*(fahrenheit - 32)";
		postfix = InfixToPostfix(infix);
		Console.WriteLine("InFix   :\t" + infix);
        Console.WriteLine("PostFix :\t" + postfix);
		
		infix = "3.1123123 * diameter";
		postfix = InfixToPostfix(infix);
		Console.WriteLine("InFix   :\t" + infix);
        Console.WriteLine("PostFix :\t" + postfix);
		
		infix = "pi*r^2";
		postfix = InfixToPostfix(infix);
		Console.WriteLine("InFix   :\t" + infix);
        Console.WriteLine("PostFix :\t" + postfix);
		
		infix = "a + b * c ^ d - e";
		postfix = InfixToPostfix(infix);
		Console.WriteLine("InFix   :\t" + infix);
        Console.WriteLine("PostFix :\t" + postfix);
		
		infix = "a ^ b + c * d";
		postfix = InfixToPostfix(infix);
		Console.WriteLine("InFix   :\t" + infix);
        Console.WriteLine("PostFix :\t" + postfix);				
		
	}
	
	public static string InfixToPostfix(string exp)
    {
        String result = String.Empty;
        Stack<char> stack = new Stack<char>();
         
        for (int i = 0; i<exp.Length; ++i)
        {
            char c = exp[i];
            if (Char.IsLetterOrDigit(c) || Char.IsWhiteSpace(c) || c == '.')
                result += c;
			
            else if (c == '(')
                stack.Push(c);
             
            else if (c == ')')
            {
                while (stack.Count!=0 && stack.Peek() != '(')
                    result += " " + stack.Pop();
                
				//Close () and clean from stack
                if (stack.Count!=0 && stack.Peek() != '(')
                    return "Invalid Expression"; // invalid expression                
                else
                    stack.Pop();
            }
            else // an operator is encountered
            {
                while (stack.Count!=0  && precedence(c) <= precedence(stack.Peek()))
				{
                    result += " " + stack.Pop();
				}
				result += " ";
                stack.Push(c);
				
            }
        }
        while (stack.Count!=0 )
            result += " " + stack.Pop();
      
        //return result.Trim();
		return System.Text.RegularExpressions.Regex.Replace(result,@"\s+"," ");
    }
	
    public static int precedence(char operand)
    {
        switch (operand)
        {
        case '+':
        case '-':
            return 1;
      
        case '*':
        case '/':
            return 2;
      
        case '^':
            return 3;
        }
        return -1;
    }
}