' See what the new value would be
' if this change is allowed.
Private Function NewValue(ByVal KeyValue As Integer, ByVal _
    is_delete As Boolean) As String
' Useful ASCII codes.
Const ASC_CTRL_V = 22
Const ASC_CTRL_X = 24
Const FIRST_ASC = 32    ' 1st visible char.
Const LAST_ASC = 126    ' Last visible char.

Dim old_txt As String
Dim part1 As String
Dim part2 As String
Dim part3 As String

    old_txt = txtInput.Text

    ' Find the pieces before and after the selection.
    part1 = Left$(old_txt, txtInput.SelStart)
    part3 = Right$(old_txt, Len(old_txt) - _
        txtInput.SelStart - txtInput.SelLength)

    ' Calculate the new text.
    part2 = ""
    If is_delete Then
        ' This is a delete character.
        ' If no text is selected, delete the
        ' following character (if there is one).
        If (txtInput.SelLength <= 0) And _
           (Len(part3) > 0) _
        Then
            part3 = Mid$(part3, 2)
        End If

        ' If text is selected or part3 is blank,
        ' just leave part2 = "".
    Else
        ' This is not a delete character.
        ' See what it is.
        Select Case KeyValue
            Case vbKeyBack      ' Backspace.
                ' If no text is selected, delete the
                ' previous character (if there is one).
                If (txtInput.SelLength <= 0) And _
                   (Len(part1) > 0) _
                Then
                    part1 = Left$(part1, Len(part1) - 1)
                End If
                ' If text is selected or part1 is
                ' blank, leave part2 = "".

            Case ASC_CTRL_V
                ' Paste the clipboard's text over
                ' the selected text.
                part2 = Clipboard.GetText(vbCFText)

            Case ASC_CTRL_X
                ' Leave part2 = "".

            Case FIRST_ASC To LAST_ASC
                ' Use the visible character typed
                ' for the middle string.
                part2 = Chr$(KeyValue)

            Case Else
                ' Assume other non-visible keys
                ' like ^C will not change the text.
                NewValue = old_txt
                Exit Function
        End Select
    End If

    ' Build the result.
    NewValue = part1 & part2 & part3
End Function

' Watch for Delete keys.
Private Sub txtInput_KeyDown(KeyCode As Integer, Shift As _
    Integer)
Dim new_value As String
Dim Cancel As Boolean

    If KeyCode = vbKeyDelete Then
        ' See what the new value would be.
        new_value = NewValue(KeyCode, True)

        ' Tell the program.
        RaiseEvent BeforeChange(new_value, Cancel)
        If Cancel Then KeyCode = 0
    End If
End Sub

' Validate key presses.
Private Sub txtInput_KeyPress(KeyAscii As Integer)
Dim new_value As String
Dim Cancel As Boolean

    ' See what the new value would be.
    new_value = NewValue(KeyAscii, False)

    ' Tell the program.
    RaiseEvent BeforeChange(new_value, Cancel)
    If Cancel Then KeyAscii = 0
End Sub

Private Sub PreviewText1_BeforeChange(ByVal new_value As _
    String, Cancel As Boolean)
    Cancel = _
        (MsgBox("Accept new value """ & new_value & """?", _
            vbQuestion Or vbYesNo, "Accept Change?") _
        = vbNo)
End Sub