TextBox Validator Function

A while back I wrote the following function becase I had a need to restrict the way a user is to have entered data into a text box. This function allows the programmer the ability to control what type of alphanumeric character is entered into a text box at a particular postion in the text box.

VB Code:

Private Function ValidateKey(TextBoxName As TextBox, Keyascii As Integer) As Integer
Dim strTagFilter As String
Dim intPosition As Integer
 
If Keyascii = 8 Then
    Exit Function
Else
    If TextBoxName.Tag & "" = "" Then
        Exit Function
    Else        intPosition = Len(TextBoxName.Text) + 1 'Sets up the Tag pointer
        strTagFilter = Mid(TextBoxName.Tag, intPosition, 1) 'Gets the Validator Code
        
        If intPosition > Len(TextBoxName.Tag) Then 'Only allows the same number of characters
                                          'that are contained in the Tag Property
            
            Keyascii = 0  'If so doesn't allow anymore characters
        Else
            TextBoxName.SelStart = intPosition 'Positions alphanumeric
                                                'character after previous
                                                'alphanumeric character
            Select Case strTagFilter
                Case "A", "a" 'Only allows Letters
                    If (Keyascii >= 65 And Keyascii <= 90) Or _
                            (Keyascii >= 97 And Keyascii <= 122) Then
                        
                            Keyascii = Keyascii
                        If strTagFilter = "A" Then 'Capitalize the letter if "A"
                           If (Keyascii >= 97 And Keyascii <= 122) Then
                                Keyascii = Keyascii - 32 ' Capitalize the first charater
                           End If
                        End If
                    Else
                        Keyascii = 0 'invalid character
                    End If
                    
                Case "0"    'Only allows numbers
                    If (Keyascii < 48 Or Keyascii > 57) Then
                        Keyascii = 0 'invalid character
                    End If
                Case Else
                    If Keyascii <> Asc(strTagFilter) Then 'Only allows the same alphanumeric
                                                          'Character (i.e. /-_=+&*()#$@!{}|\)
                       Keyascii = 0 'invalid character
                    End If
            End Select
        End If
    End If
End If
End Function

... and you call it like this ...

VB Code:

Private Sub Text1_KeyPress(Keyascii As Integer)
    Call ValidateKey(Me.Text1, Keyascii)
End Sub

So to allow the following number to be entered into a text box US35-39234B then enter th following into that textboxes Tag Property: AA00-00000A

here's a one-liner that does a very similar thing (only supports uppercase though):

VB Code:

' Tag property is set to "AA00-00000A"
Private Sub Text1_KeyPress(KeyAscii As Integer)
    If Not KeyAscii = vbKeyDelete Then KeyAscii = -KeyAscii * (Text1.Text & Chr$(KeyAscii) Like Replace(Left$(Text1.Tag, Len(Text1.Text) + 1), "A", "[A-Z]"))
End Sub

i wouldn't seriously suggest that someone actually uses this, it's just another way of approaching the problem.