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.

I'd use a For..Next and a Select statement.

For ilN = 1 to Len(Activecell.value)

Select case mid(Activecell.value,ilN,1)
Case "1", "2", "3", "4", "5", "6", "7", "8", "9", "-"
' Do something.
Case Else
' Do something else.
End Select

Next ilN

I use this sort of thing all the time for stuff... Eg Stipping control
characters out.

ok not sure if this has been answered or not but i think you should be able
to shorten it if you use the instr function

aircode

if not instr(cch, "0123456789-") then
MY_FUNCTION
end if

should cover it

end aircode