VB.net: Error with mid function - vb.net

Question

Im making a program that will only accept a userID which is in a format like Hal456, the first one is in uppercase, next two lowercase and the last three numbers.
I tried going through the program step by step and it fails at the second IF statement.
Sub Main()
Dim userid As String
Dim flag As Boolean
flag = False
Console.WriteLine("Input userID to check format")
userid = "Hal123"
Dim a, b, c, d, e, f As Integer
a = Asc("a")
b = Asc("z")
c = Asc("A")
d = Asc("Z")
e = Asc("0")
f = Asc("9")
If Len(userid) = 6 Then
If Asc(Left(userid, 1)) > c And Asc(Left(userid, 1)) < d Then
If Asc(Mid(userid, 2, 1)) > a And Asc(Mid(userid, 2, 1)) < b Then
If Asc(Mid(userid, 3, 1)) > a And Asc(Mid(userid, 3, 1)) < b Then
If Asc(Mid(userid, 4, 1)) > e And Asc(Mid(userid, 4, 1)) < f Then
If Asc(Mid(userid, 5, 1)) > e And Asc(Mid(userid, 5, 1)) < f Then
If Asc(Mid(userid, 6, 1)) > e And Asc(Mid(userid, 6, 1)) < f Then
flag = True
Console.WriteLine("Format of userID is correct")
End If
End If
End If
End If
End If
End If
End If
If flag = False Then Console.WriteLine("Format is not correct")
Console.ReadLine()
End Sub

Answer 1

I see the problem with the code. It is a simple fix. Just change all the "<" to "<=" and all the ">" to ">=" without quotes.
Reason:
Your Program will not allow "a" and "z" and "0" and "9" to be part of the userid.
Changing it to greater than or equal to solves this.
Hope this helps!

Answer 2

Try this using Regular Expressions:
Sub Main()
Dim userid As String
Dim flag As Boolean
Console.WriteLine("Input userID to check format")
userid = "Hal123"
flag = CheckUserID(userid)
If flag = False Then Console.WriteLine("Format is not correct")
Console.ReadLine()
End Sub
Private Function CheckUserID(userId As String) As Boolean
Dim regex As Regex = New Regex("([A-Z]){1}([a-z]){2}([0-9]){3}")
Dim match As Match = regex.Match(userId)
Return match.Success
End Function

Answer 3

How about using REGEXP for solving this? I think that should be quite straight Forward.
Something like:
string pattern = #"[A-Z][a-z]{2}\d{3}";
Regex rx = new Regex(pattern);
string uid = "";
do
{
Console.WriteLine("Please insert userID:");
uid = Console.ReadLine();
bool check = rx.IsMatch(uid);
} while (!rx.IsMatch(uid));