using static System.Console;

using System.Text;

​

public class Program {

    public static void Main() {

        WriteLine("123.456.789/ 0001-99X".Strip());

        WriteLine("(19)9-98/754?283 A".Strip());

    }

}

​

public static class StringExt {

    public static string Strip(this string str) {

        var sb = new StringBuilder(str.Length);

        foreach (var chr in str) if (char.IsDigit(chr)) sb.Append(chr);

        return sb.ToString();

    }

}