How to properly access the value of a capture group in C# regex?

I have the following code:

using System;
using System.Text.RegularExpressions;

public class Test
{
   public static void Main()
   {
      var r = new Regex(@"_(\d+)$");
      string new_name = "asdf_1";

      new_name = r.Replace(new_name, match =>
      {
         Console.WriteLine(match.Value);
         return match.Value;
         //return (Convert.ToUInt32(match.Value) + 1).ToString();
      });

      //Console.WriteLine(new_name);
   }
}

I expect match.Value to be 1, but it is printing as _1. What am I doing wrong?