using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Text.RegularExpressions;

namespace ConsoleApplication3

{

    class Program

    {

        static void Main(string[] args)

        {

string s = "<ConnectString>Data Source=asdf, Database=fees</ConnectString>";

            Regex r = new Regex("(?<=<ConnectString>).*?(?=</ConnectString>)", RegexOptions.IgnoreCase);

            string result = r.Replace(s, "新字符串");

            Console.WriteLine(result);

            Console.ReadKey();

        }

    }

}