Validation based on data entry

I need to have 2 validation rules in my spreadsheet:

In a cell within the range A3:A20 the user must type in a username either 7 or 8 characters (mixture letters and numbers) but must start with N or S or error displayed and user cannot proceed.

When this cell has validated, on entering data into a cell within the range G3:G20, if the user has typed in a username in a cell within the range A3:A20 that starts with N, then they must type in a username starting with N and similarly if the username starts with S, this also needs to start with S.

I've written an if else statement but would be grateful for any help. Hope this makes sense.

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)

If (Left(A3, 1) = "N") Then
response1 = MsgBox("username must start with N", vbOKOnly)
ElseIf (Left(A3, 1) = "S") Then
response2 = MsgBox("username must start with S", vbOKOnly)
End If
End Sub