This is a far better proposition:
Code:
Function udfIsInt($i)
	$udfIsInt=0
	While $i
		If Not InStr("0123456789",Left($i,1)) Exit 1 EndIf
		$i=SubStr($i,2)
	Loop
	$udfIsInt=1
	Exit 0
EndFunction


This will return true for "" and "0001", but will return false for "123x456".

Note, you can very simply convert this to a general "check that only these values are present" function by making the "valid" string an additional optional parameter.