Well it's not as pretty, but this one seems to work.
 Code:
? isNum("01234")
? isNum("12345")
? isNum("01230")
? isNum("0123a")
? isNum("a1234")

get $

Function isNum($n)
   Dim $x,$c,$count
   For $x=1 to Len($n)
      $c = SubStr($n,$x,1)
      If $c = "0" OR Val($c)
         $count = $count+1
      Endif
   Next
   $isNum = IIF(Len($n)=$count, 1, 0)
EndFunction