Parsing a postfix expression in C#, my naive approach

kashu

using System;
using System.Text.RegularExpressions;
using System.Collections.Generic;
namespace Postfix
{
class Parser
{
public delegate int BinOp(int a, int b);
static int add(int a, int b) { return a + b;}
static int sub(int a, int b) { return a - b; }
static int mul(int a, int b) { return a * b; }
static int div(int a, int b) { return a / b; }
static int parseExp(string s)
{
Dictionary<string, BinOp> oper = new Dictionary<string, BinOp>();
List<int> stack =new List<int>();
oper["+"] = add;
oper["-"] = sub;
oper["*"] = mul;
oper["/"] = div;
Regex rxnum = new Regex(@"/G/s*-?([0-9]+)/s+");
Regex rxop = new Regex(@"/G(/+|/-|/*|//)");
int start = 0;
do
{
Match m = rxnum.Match(s, start);
if (m.Success)
{
stack.Add(Int32.Parse(m.ToString()));
}
else
{
m = rxop.Match(s, start);
if (m.Success)
{
int c = stack.Count;
if (c < 2) throw new Exception("Invalid expression: out of stack.");
stack[c - 2] = oper[m.ToString()](stack[c - 2], stack[c - 1]);
stack.RemoveAt(c - 1);
}
else break;
}
start = start + m.Length;
} while (true);
if (stack.Count != 1) throw new Exception("Invalid expression: more than one result on stack.");
if (start != s.Length) throw new Exception("Invalid expression: unrecognized token.");
return stack[0];
}
static void Main()
{
string exp = "23 5 - 12 *";
Console.WriteLine("expression '{0}' result is '{1}'", exp,  Parser.parseExp(exp));
}
}
}

　　
或
　　namespace Postfix
{
class Parser
{
static Dictionary<string, Func<int, int, int>> operators;
static Regex parserEngine = new Regex(@"/G(/s+(?<op>[-+*/])|/s*(?<value>-?[0-9]+))", RegexOptions.ExplicitCapture);
static Parser()
{
operators = new Dictionary<string, Func<int, int, int>>();
operators["+"] = (a, b) => a + b;
operators["-"] = (a, b) => a - b;
operators["*"] = (a, b) => a * b;
operators["/"] = (a, b) => a / b;
}
static int parseExp(string s)
{
Stack<int> stack = new Stack<int>();
foreach(Match match in parserEngine.Matches(s))
{
if(match.Success)
{
foreach(KeyValuePair<string, string> m in match.Groups)
{
if(m.Key == "op")
{
if(stack.Count < 2)
throw new Exception("Invalid expression: out of stack.");
int b = stack.Pop();
int a = stack.Pop();
stack.Push(operators[m.Value](a, b));
}
else if(m.Key == "value")
stack.Push(int.Parse(m.Value));
}
}
}
if(stack.Count != 1)
throw new Exception("Invalid expression: not exactly one result on stack.");
return stack.Pop();
}
static void Main()
{
string exp = "23 5 - 12 *";
Console.WriteLine("expression '{0}' result is '{1}'", exp, Parser.parseExp(exp));
}
}
}