src/Pure/General/symbol.ML

   val is_digit: symbol -> bool
   val is_quasi: symbol -> bool
   val is_quasi_letter: symbol -> bool
   val is_letdig: symbol -> bool
   val is_blank: symbol -> bool
+  val is_identifier: symbol -> bool
   val is_symbolic: symbol -> bool
   val is_printable: symbol -> bool
   val length: symbol list -> int
   val strip_blanks: string -> string
   val beginning: symbol list -> string

 
 fun is_printable s =
   size s = 1 andalso ord space <= ord s andalso ord s <= ord "~" orelse
   is_symbolic s;
 
+fun is_identifier s =
+  case (explode s) of
+      [] => false
+    | c::cs => is_letter c andalso forall is_letdig cs;
 
 fun sym_length ss = foldl (fn (n, s) =>
   (if not (is_printable s) then 0 else
     (case Library.try String.substring (s, 2, 4) of
       Some s' => if s' = "long" orelse s' = "Long" then 2 else 1