def der (r: Rexp, c: Char) : Rexp = r match {+ −
case NULL => NULL+ −
case EMPTY => NULL+ −
case CHAR(d) => if (c == d) EMPTY else NULL+ −
case ALT(r1, r2) => ALT(der(r1, c), der(r2, c))+ −
case SEQ(r1, r2) => + −
if (nullable(r1)) ALT(SEQ(der(r1, c), r2), der(r2, c))+ −
else SEQ(der(r1, c), r2)+ −
case STAR(r) => SEQ(der(r, c), STAR(r))+ −
}+ −
+ −
def ders (s: List[Char], r: Rexp) : Rexp = s match {+ −
case Nil => r+ −
case c::s => ders(s, der(c, r))+ −
}+ −
+ −