C#，如何程序使用正则表达式如何使用匹配的位置的结果修改匹配到的值

程序代码使用正则表达式如何修改匹配到的值：

代码一：

using System;
using System.Text.RegularExpressions;
public class Example
{
    public static void Main()
    {
        string input = "1851 1999 1950 1905 2003";
        string pattern = @"(?<=19)\d{2}";
        //\b(\w+)(\s)(\w+)\b    $3$2$1    "one two"
        input="one two";
        pattern=@"\b(\w+)(\s)(\w+)\b";
        Console.WriteLine(Regex.Replace(input,pattern,"1.$3 $2 2.$1", RegexOptions.Compiled | RegexOptions.IgnoreCase));
        foreach (Match match in Regex.Matches(input, pattern))
        Console.WriteLine(match.Value);
        //\b(?< word1>\w+)(\s)(?< word2>\w+)\b ${word2} ${word1} "one two"
        input="one two";
        pattern=@"\b(?<word1>\w+)(\s)(?<word2>\w+)\b";
        Console.WriteLine(Regex.Replace(input,pattern,"${word2} ${word1}", RegexOptions.Compiled | RegexOptions.IgnoreCase));
        foreach (Match match in Regex.Matches(input, pattern))
                  Console.WriteLine(match.Value);
    }
}

　　

　　代码二：

using System;
using System.Text.RegularExpressions;

namespace RegExApplication
{
   class Program
   {
      static void Main(string[] args)
      {
        string s = "<img aa=bb cc=dd src=\"\img\1\111.jpg\" />变成 <img src=\"http://aaa/bbbcca.jpg\" /><img alt=\"hello\" src=\"222.jpg\" /><img src=\"http://www.w3dev.cn/logo.jpg\" />";
        Console.WriteLine(s);
        Console.WriteLine("****************************");
        s = Regex.Replace(s, "(<img[\\s\\S]+?)src=([\"'])(?!(https?://))([^\"']+)", "$1src=$2http://aaaa/$4", RegexOptions.Compiled | RegexOptions.IgnoreCase);
        Console.WriteLine(  s );
      }
   }