type value_constraint = Regex of string | External of string * string

exception Bad_validator of string

let validate_value validators value_constraint value =
    match value_constraint with
    | Regex s ->      
        (try 
           let _ = Pcre.exec ~pat:s value in true
       with Not_found -> false)
    | External (t, c) ->
        try
            let validator = Hashtbl.find validators t in
            let result = Unix.system (Printf.sprintf "%s %s %s" validator c value) in
            match result with
            | Unix.WEXITED 0 -> true
            | _ -> false
        with Not_found -> raise (Bad_validator t)

(* If no constraints given, consider it valid.
   Otherwise consider it valid if it satisfies at least
   one constraint *)
let validate_any validators constraints value =
    let rec aux validators constraints value  =
        match constraints with
        | [] -> false
        | c :: cs -> if validate_value validators c value then true
                     else aux validators cs value
    in
    match constraints with
    | [] -> true
    | _ -> aux validators constraints value