System.Text.RegularExpressions.RegexCharClass.NegateCategory C# (CSharp) Method

        private static string NegateCategory(string category) {
            if (category == null)
                return null;

            StringBuilder sb = new StringBuilder(category.Length);

            for (int i=0; i<category.Length; i++) {
                short ch = (short) category[i];
                sb.Append( (char) -ch);
            }
            return sb.ToString();
        }