System.Text.RegularExpressions.RegexParser.HexDigit C# (CSharp) Method

        internal static int HexDigit(char ch)
        {
            int d;

            if ((uint)(d = ch - '0') <= 9)
                return d;

            if ((uint)(d = ch - 'a') <= 5)
                return d + 0xa;

            if ((uint)(d = ch - 'A') <= 5)
                return d + 0xa;

            return -1;
        }