Parsing a postfix expression in C#, my naive approach

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));
}
}
}