14 case object ONE extends Rexp                     // matches an empty string

    14 case object ONE extends Rexp                     // matches an empty string

    15 case class CHAR(c: Char) extends Rexp            // matches a character c

    15 case class CHAR(c: Char) extends Rexp            // matches a character c

    16 case class ALT(r1: Rexp, r2: Rexp) extends Rexp  // alternative

    16 case class ALT(r1: Rexp, r2: Rexp) extends Rexp  // alternative

    17 case class SEQ(r1: Rexp, r2: Rexp) extends Rexp  // sequence

    17 case class SEQ(r1: Rexp, r2: Rexp) extends Rexp  // sequence

    18 case class STAR(r: Rexp) extends Rexp            // star

    18 case class STAR(r: Rexp) extends Rexp            // star

    19 

    20 

    20 // nullable function: tests whether a regular 

    21 // nullable function: tests whether a regular 

    21 // expression can recognise the empty string

    22 // expression can recognise the empty string

    22 def nullable(r: Rexp) : Boolean = r match {

    23 def nullable(r: Rexp) : Boolean = r match {

    23   case ZERO => false

    24   case ZERO => false