(** {2 Regular Expression Fragments} *) val xnx : string -> string (** Return a regular expression that matches: optional x - non-x with embedded x - optional x *) val xonx : string -> string (** optional x - optional non-x with embedded x - optional x *) val wnw : unit -> string (** white - nonwhite with embedded white - white *) val wonw : unit -> string (** white - optional nonwhite with embedded white - white *)