using System;

using System.Text;

using System.Text.RegularExpressions;

using System.Linq;

public class Program

{

    public static void Main()

    {

        // Input variables

        string mask = @"ab*\*, hello ?or\?maybe@benteler.com, anychar:@";

        char[] exceptionCharacters = {'*', '#', '?'};

        StringBuilder escapedMask = new StringBuilder("", mask.Length * 2);

        bool isEscaped = false;

        // Iterate through characters of mask

        for (int i = 0; i < mask.Length; i++)

        {

            if (!isEscaped && mask[i].Equals('\\'))

            {

                // If backslash is found, we take a note of it and jump to next character

                isEscaped = true;

                continue;

            }

            else if (exceptionCharacters.Contains(mask[i]))

            {

                // Characters with special meaning (and their preceding backslash) will be exempt from escaping

                escapedMask.Append((isEscaped ? @"\": "") + mask[i].ToString());

            }   

            else

            {

                // Other characters must be escaped as usual

                escapedMask.Append(Regex.Escape((isEscaped ? @"\": "") + mask[i].ToString()));

            }

            // Reset isEscaped flag

            if (isEscaped)

                isEscaped = false;

        }

        Console.WriteLine("Original:     " + mask);

        Console.WriteLine("Regex.Escape: " + Regex.Escape(mask));

        Console.WriteLine("Our.Escape:   " + escapedMask.ToString());

    }

}