type token =
    | DoubleUnderscore
    | Underscore
    | DoubleTimes
    | Times
    | EmptyToken
    | Literal of string

let (|StartsWith|_|) (p:string) (s:string) =
    if s.StartsWith(p) then
        Some(s.Substring(p.Length))
    else
        None

let buildToken charlist =
    match charlist with
    | [] -> EmptyToken
    | _ -> Literal(System.String.Concat<char> (List.rev charlist))

let rec tokenize str =
    let rec helper str litAccu accu =
        match str with
        | StartsWith "__" _ -> helper (str.Substring 2) [] (DoubleUnderscore :: (buildToken litAccu) :: accu)
        | StartsWith "**" _ -> helper (str.Substring 2) [] (DoubleTimes :: (buildToken litAccu) :: accu)
        | StartsWith "_" _ -> helper (str.Substring 1) [] (Underscore :: (buildToken litAccu) :: accu)
        | StartsWith "*" _ -> helper (str.Substring 1) [] (Times :: (buildToken litAccu) :: accu)
        | "" -> (buildToken litAccu) :: accu
        | _ -> helper (str.Substring 1) (str.[0] :: litAccu) accu
    helper str [] [] |> List.rev |> List.filter (fun x -> x <> EmptyToken)

tokenize "foo_bar__**baz!!a*sereje"