gistfile1.fs

open FParsec
open FParsec.CharParsers

let toInt ch = (int ch) - (int '0')

let parser = parse {
  let! head = digit |>> toInt
  do! digit |>> ignore
  do! digit |>> ignore
  do! digit |>> ignore
  do! digit |>> ignore
  let! sixth = digit |>> toInt
  match head with
  | 8 | 9 ->
      if 1 <= sixth && sixth <= 5 then
        return (head, sixth)
      else
        return! fail "oops!"
  | _ ->
      return (head, sixth)
}

let tryParse str =
  match run (parser .>> eof) str with
  | Success (res, _, _) -> Some res
  | Failure _ -> None

let validate str =
  match tryParse str with
  | Some x -> printfn "valid! %A" x
  | None -> printfn "invalid..."

[<EntryPoint>]
let main argv = 
  validate "923451" // ok
  validate "923455" // ok
  validate "923456" // ng
  validate "123456" // ok
  0

gistfile2.fs

// 別解:アクティブパターンで
let (|Head|) (str: string) = str.[0] |> string |> int
let (|Sixth|) (str: string) = str.[5] |> string |> int

let validate str =
  match str with
  | Head head when head = 8 || head = 9 ->
      match str with
      | Sixth sixth when 1 <= sixth && sixth <= 5 ->
          printfn "valid! %A" (head, sixth)
      | _ ->
          printfn "invalid..."
  | Head head ->
      printfn "valid! %A" (head, str.[5] |> string |> int)

[<EntryPoint>]
let main argv = 
  validate "923451" // ok
  validate "923455" // ok
  validate "923456" // ng
  validate "123456" // ok
  0