I don't use ocamlyacc,  but I do have lots of ocamllex lexers that need
to be reentrant. I don't remember if I've  posted that trick to the list
already, but this is what you can do:

In your opening section of the lexer:

{
type t = <something to store the data>

let create_data () = ....    (* unit -> t *)

}

and then for all rules, use

rule somerule = parse
 | somepattern { (fun lexdata -> action) }


And, when invoking a lexer function or entry point,  you need to pass the
additionnal argument, a in
  somerule lexbuf lexdata

--f