Luhn implementation F#

Luhn.fsx

let luhn (inp:string) =
    let algo (s:string) = 
        Regex.Replace(s, "[^0-9]", "").ToCharArray()
        |> Array.rev
        |> Seq.map(int << string)
        |> Seq.fold (fun (fldr,weight) num -> fldr + (num * weight) / 10 + (num * weight) % 10, weight % 2 + 1) (0,2)
        |> fst
    match inp with
    | "" -> 0
    | _ -> 10 - (algo inp) % 10