+fun isHexDigit ch = Char.isDigit ch orelse (ord ch >= ord #"a" andalso ord ch <= ord #"f")
+
+fun validIpv6 s =
+ let
+ val fields = String.fields (fn ch => ch = #":") s
+
+ val empties = foldl (fn ("", n) => n + 1
+ | (_, n) => n) 0 fields
+
+ fun noIpv4 maxLen =
+ length fields >= 2
+ andalso length fields <= maxLen
+ andalso empties <= 1
+ andalso List.all (fn "" => true
+ | s => size s <= 4
+ andalso CharVector.all isHexDigit s) fields
+
+ fun hasIpv4 () =
+ length fields > 0
+ andalso
+ let
+ val maybeIpv4 = List.last fields
+ val theRest = List.take (fields, length fields - 1)
+ in
+ validIp maybeIpv4 andalso noIpv4 6
+ end
+ in
+ noIpv4 8 orelse hasIpv4 ()
+ end
+