I just made some modifications to Joel post. Since you only want Letters and
Numbers in the textbox it will probably be very useful to use a
AfterUpdate_Event. This will test the Text in the Textbox once the Textbox
looses its focus. I added a message box as well.
Private Sub TextBox1_AfterUpdate()
Dim TestStr As String
Dim I As Integer
Dim TestChar As String
TestStr = TextBox1.Text
For I = 1 To Len(TestStr)
TestChar = UCase(Mid(TestStr, I, 1))
If Not IsNumeric(TestChar) Then
If Asc(TestChar) < Asc("A") Or _
Asc(TestChar) > Asc("Z") Then
MsgBox "You need to enter Letters & Numbers only!", vbExclamation
Exit For
End If
End If
Next I
End Sub
Hope this helps!