using System;

public class Program

{

    public static void Main()

    {

        string input = @"\\machinename\folder1\folder2";

        string pattern = @"^(\\+)([a-z0-9_!\\]+)+$";

        string replacement = @"\$2";

        string result = System.Text.RegularExpressions.Regex.Replace(input, pattern, replacement);

        Console.WriteLine(result);

        // RegexPal: http://www.regexpal.com/?fam=96434

    }

}