using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

​

      string pattern = @"(""[^"",]+),([^""]+"")";

      string input ="7/12/2017,100,\"Direct Credit 421520 LIANGLI ZHONG Bonnie Zhong, Kind\",50753.51";

      string replacement = "$1$2";

      Regex rgx = new Regex(pattern);

      string result = rgx.Replace(input, replacement);

​

      Console.WriteLine("Original String:    '{0}'", input);

      Console.WriteLine("Replacement String: '{0}'", result);          

    }

}