Thanks, Shadez and Mile-O-Phile, for your assistance.
This is what I was looking for:
If InStr(1, Me.TxtRename, "!", vbTextCompare) Or _
InStr(1, Me.TxtRename, ".", vbTextCompare) Or _
InStr(1, Me.TxtRename, "`", vbTextCompare) Then
MsgBox "The characters:" & vbCrLf & "! . `" &...