9 

    10 def der (c: Char, r: Rexp) : Rexp = r match {

    11   case NULL => NULL

    12   case EMPTY => NULL

    13   case CHAR(d) => if (c == d) EMPTY else NULL

    14   case ALT(r1, r2) => ALT(der(c, r1), der(c, r2))

    15   case SEQ(r1, r2) => 

    16     if (nullable(r1)) ALT(SEQ(der(c, r1), r2), der(c, r2))

    17     else SEQ(der(c, r1), r2)

    18   case STAR(r) => SEQ(der(c, r), STAR(r))

    19 }

    20 

    21 def ders (s: List[Char], r: Rexp) : Rexp = s match {

    22   case Nil => r

    23   case c::s => ders(s, der(c, r))

    24 }

    25 

    26 def matches(r: Rexp, s: String) : Boolean = 

    27   nullable(ders(s.toList, r))