using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string ean = "735999119039119036";

        string pattern = "^735999[0-9]{5}([0-9]{6})[0-9]$";

        string output = Regex.Replace(Regex.Replace(ean, "^735999[0-9]{5}", ""), "[0-9]$", "");

        Console.WriteLine(output);

        string output2 = Regex.Replace(ean, pattern, m => m.Groups[1].Value);

        Console.WriteLine(output2);

        string pattern2 = "^735999[0-9]{5}(?=[0-9]{6})";

        string output3 = Regex.Replace(ean, pattern2, "");

        Console.WriteLine(output3);

        string pattern3 = "(^735999[0-9]{5})";

        string output4 = Regex.Replace(ean, pattern3, "");

        Console.WriteLine(output3);

        Console.WriteLine(ReplaceTwoSteps(ean, "^735999[0-9]{5}", "", "[0-9]$", ""));

        Console.WriteLine(Replacer(ean, pattern, "\\1"));

    }

    private static string ReplaceTwoSteps(string input, string pattern1, string replace1, string pattern2, string replace2)

    {

        return Regex.Replace(Regex.Replace(input, pattern1, replace1), pattern2, replace2);

    }

    private static string Replacer(string input, string pattern, string replace)

    {

        return Regex.Replace(input, pattern, m => EvaluateMatch(m, replace));

    }

    private static string EvaluateMatch(Match m, string replace)

    {

        for (int i = 1; i < m.Groups.Count; i++)

        {

            var g = m.Groups[i];

            replace = replace.Replace("\\" + i, g.Value);

        }

        return replace;

    }

}