using System;

using System.Text;

using System.Text.RegularExpressions;

using System.Linq;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        ProcessText(@"D'Ambrosi Rebecca");

        ProcessText(@"Evans De'Queela");

        ProcessText(@"Gerald Preiß");

​

    }

    private static void ProcessText(string text)

    {

        var output = ConvertVowelMutation(text);

        output = RemoveDiacritics(output);

        output = RemoveIllegalChars(output);

        Console.WriteLine(output);

    }

            private static readonly (string original, string target)[] _mutatedVowels = new (string original, string target)[]

        {

            ("ä", "ae"),

            ("ö", "oe"),

            ("ü", "ue"),

            ("Ä", "Ae"),

            ("Ö", "Oe"),

            ("Ü", "Ue"),

            ("ß", "ss")

        };

            private static string ConvertVowelMutation(string text)

        {

            if (string.IsNullOrWhiteSpace(text))

                return text;

​

            foreach (var mutatedVowel in _mutatedVowels) 

                text = text.Replace(mutatedVowel.original, mutatedVowel.target);

​

            return text;

        }

            private static string RemoveDiacritics(string text)

        {

            if (string.IsNullOrWhiteSpace(text))

                return text;

​

            text = text.Normalize(NormalizationForm.FormD);

            var chars = text.Where(c => CharUnicodeInfo.GetUnicodeCategory(c) != UnicodeCategory.NonSpacingMark).ToArray();

            return new string(chars).Normalize(NormalizationForm.FormC);

        }

        private static readonly Regex _legalCharRegex = new Regex(@"([0-9]|[a-z]| |-|_)*", RegexOptions.IgnoreCase);

        private static string RemoveIllegalChars(string text)

        {

            var r = _legalCharRegex.Matches(text);

​

            var ret = string.Empty;

            foreach (Match match in r)

            {

                ret += match.Value;

            }

            return ret;

        }

}