1 abstract class Rexp

     1 def OPT(r: Rexp) = ALT(r, EMPTY)

     3 case object NULL extends Rexp

     3 def NTIMES(r: Rexp, n: Int) : Rexp = n match {

     4 case object EMPTY extends Rexp

     4   case 0 => EMPTY

     5 case class CHAR(c: Char) extends Rexp

     5   case 1 => r

     6 case class ALT(r1: Rexp, r2: Rexp) extends Rexp

     6   case n => SEQ(r, NTIMES(r, n - 1))

     7 case class SEQ(r1: Rexp, r2: Rexp) extends Rexp

     7 }

     8 case class STAR(r: Rexp) extends Rexp

     8