using System;

using System.Text.RegularExpressions;

​

public class Program

{

    private static Regex _invalidXMLChars = new Regex(@"(?<![\uD800-\uDBFF])[\uDC00-\uDFFF]|[\uD800-\uDBFF](?![\uDC00-\uDFFF])|[\x00-\x08\x0B\x0C\x0E-\x1F\x7F-\x9F\uFEFF\uFFFE\uFFFF]", RegexOptions.Compiled);

​

    public static void Main()

    {

        var text = "assd•abv";

        Console.WriteLine(_invalidXMLChars.IsMatch(text));

    }

}