Imports System.Text.RegularExpressions
...
Private Function MultiplyByTwoEvaluator(ByVal match As Match) As String
    Return (Int32.Parse(match.Value) * 2).ToString()
End Function
...
Dim input As String = "FG4511H126"
input = Regex.Replace(input, "\d", (AddressOf MultiplyByTwoEvaluator))

byte[] BB = Encoding.Default.GetBytes(s);
...
if (BB[Q] >= 48 && BB[Q] <= 57)

private static void Main(string[] args)
{
	const string digits = "0123456789";
	byte[] expectedDigitCodes = Encoding.ASCII.GetBytes(digits);
	DumpArray(expectedDigitCodes);

	int errorCount = 0;

	EncodingInfo[] encodings = Encoding.GetEncodings();
	foreach (EncodingInfo ei in  encodings)
	{
		Encoding enc = ei.GetEncoding();
		byte[] digitCodes = enc.GetBytes(digits);
		if (expectedDigitCodes.Length != digitCodes.Length)
		{
			errorCount++;
			Console.WriteLine("Legnth is different for encoding " + ei.Name);
			DumpArray(digitCodes);
			Console.WriteLine();
		}
		else
		{
			errorCount++;
			for (int i = 0; i < digitCodes.Length; i++)
			{
				if (digitCodes[i] != expectedDigitCodes[i])
				{
					Console.WriteLine("Unexpected values for " + ei.Name);
					DumpArray(digitCodes);
					Console.WriteLine();
					break;
				}
			}
		}
	}

	Console.WriteLine("Total encodings: {0}, errors: {1}", encodings.Length, errorCount);
}

private static void DumpArray(byte[] bytes)
{
	foreach (var b in bytes)
	{
		Console.Write("{0:X2} ", b);
	}
	Console.WriteLine();
}