using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string test = "dsfsf Sentinel for ttsetst";

        Console.WriteLine(Regex.Replace(test, @"(\s+)(?i)s(?-i)entinel\s(?i)f(?-i)or(\s+)", string.Empty));

    }

}