You want to validate the cell  contents, doing nothing if valid, but
executing MY_FUNCTION if not?

Sub blah()
valid = True
For i = 1 To Len(ActiveCell.Value)
cch = Mid(ActiveCell.Value, i, 1)
If InStr("1234567890-", cch) = 0 Then valid = False: Exit For
Next i
If Not valid Then MY_FUNCTION
End Sub

This can be easily edited to check  for the presence of only those
characters within the double quote marks.