C#

using System;
using System.Collections;
using System.Text;
using System.Text.RegularExpressions;
using System.Collections.Generic;

namespace Infix2Postfix
{
    public class Program
    {
        static void Main(string[] args)
        {
            #region variable declarations
            //declare and initialize the string to accept an expression in infix notation
            String stringInfix = "a+b/c-d";

            StringBuilder SbPostfix = new StringBuilder();
            Stack<char> stackGeneral = new Stack<char>();
            Stack<char> stackOperator = new Stack<char>();
            char[] tempArray = stringInfix.ToCharArray();

            Array.Reverse(tempArray);
            Regex regexAlphaNum = new Regex("[a-zA-Z0-9]");
            char temp;

            #endregion

            #region logic

            foreach (char ch in tempArray)
                stackGeneral.Push(ch);

            foreach (char ch in stackGeneral)
            {
                if (regexAlphaNum.IsMatch(Convert.ToString(ch)))
                    SbPostfix.Append(ch);
                else if (stackOperator.Count == 0)
                    stackOperator.Push(ch);
                else if (stackOperator.Count != 0)
                    switch (ch)
                    {
                        case '+':
                            temp = stackOperator.Peek();
                            if (temp == '*' || temp == '/')
                            {
                                while (stackOperator.Count > 0)
                                    SbPostfix.Append(stackOperator.Pop());
                                stackOperator.Push(ch);
                            }
                            else
                                stackOperator.Push(ch);
                            break;
                        case '-':
                            temp = stackOperator.Peek();
                            if (temp == '*' || temp == '/')
                            {
                                while (stackOperator.Count > 0)
                                    SbPostfix.Append(stackOperator.Pop());
                                stackOperator.Push(ch);
                            }
                            else
                                stackOperator.Push(ch);
                            break;
                        case '*':
                            temp = stackOperator.Peek();
                            if (temp == '*' || temp == '/')
                            {
                                while (stackOperator.Count > 0)
                                    SbPostfix.Append(stackOperator.Pop());
                                stackOperator.Push(ch);
                            }
                            else
                                stackOperator.Push(ch);
                            break;
                        case '/':
                            temp = stackOperator.Peek();
                            if (temp == '*' || temp == '/')
                            {
                                while (stackOperator.Count > 0)
                                    SbPostfix.Append(stackOperator.Pop());
                                stackOperator.Push(ch);
                            }
                            else
                                stackOperator.Push(ch);
                            break;
                    }

            }
            while (stackOperator.Count > 0)
                SbPostfix.Append(stackOperator.Pop());

            #endregion

            #region output to screen
            Console.WriteLine("Equation in infix notation: {0}", stringInfix);
            Console.Write("Equation in postfix/RPN notation: {0}", SbPostfix);
            Console.ReadKey();
            #endregion

        }
    }
}