using System;

using System.Text.RegularExpressions;

using System.Text;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        string source = "Dûtch Vân dér Lînde! -'!#$%&'*+-/=?^_`{|}~";

        Console.WriteLine(CleanUpSenderPrefix(source));

    }

    private static string CleanUpSenderPrefix(string senderPrefix)

    {

        if (string.IsNullOrEmpty(senderPrefix)) return senderPrefix;

​

        // lower-case and remove diacritics (turn é into e, for example)

        var normalizedString = senderPrefix.ToLower().Normalize(NormalizationForm.FormD);

        StringBuilder sb = new StringBuilder();

​

        foreach (var c in normalizedString)

        {

            var unicodeCategory = CharUnicodeInfo.GetUnicodeCategory(c);

            if (unicodeCategory != UnicodeCategory.NonSpacingMark)

            {

                sb.Append(c);

            }

        }

        string cleanedUp = sb.ToString().Normalize(NormalizationForm.FormC);

​

        // remove anything not a-z

        cleanedUp = Regex.Replace(cleanedUp, @"[^a-z]+", "");

​

        return cleanedUp;

    }

}