如何在MatchCollection中获取匹配的索引？

using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main()
    {
        string input = "Hello, world! This is a test.";
        string pattern = @"\b\w{5}\b"; // 匹配长度为5的单词

        MatchCollection matches = Regex.Matches(input, pattern);

        foreach (Match match in matches)
        {
            Console.WriteLine("匹配项: {0}", match.Value);
            Console.WriteLine("索引: {0}", match.Index);
            Console.WriteLine();
        }
    }
}