using System;
using System.Collections;
using System.Collections.Generic;
using System.Text;
using System.Globalization;
using System.Text.RegularExpressions;

namespace Enum
{
    class Program
    {
        static void Main(string[] args)
        {
            string sql = "INSERT INTO TABLE1(col1, col2, col3, col4, col5) VALUES (,NULL, NULL, NULL, NULL, NULL,)";

            string nulls  = "NULL";
            List<int> indexes = new List<int>();
            foreach (Match match in Regex.Matches(sql, nulls))
            {
                indexes.Add(match.Index);
            }

            sql = sql.Remove(indexes[2], 4).Insert(indexes[2], "'abc'");
            Console.WriteLine(sql);
        }
    }
}