--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/progs/re2.scala	Sat Jun 15 09:23:18 2013 -0400
@@ -0,0 +1,100 @@
+    
+abstract class Rexp {
+  def simp : Rexp = this
+}
+
+case object NULL extends Rexp
+case object EMPTY extends Rexp
+case class CHAR(c: Char) extends Rexp
+case class ALT(r1: Rexp, r2: Rexp) extends Rexp {
+  override def simp = (r1.simp, r2.simp) match {
+    case (NULL, r) => r
+    case (r, NULL) => r
+    case (r, EMPTY) => if (nullable(r)) r else ALT(r, EMPTY)
+    case (EMPTY, r) => if (nullable(r)) r else ALT(r, EMPTY)
+    case (r1, r2) => if (r1 == r2) r1 else ALT(r1, r2)
+  }
+}
+case class SEQ(r1: Rexp, r2: Rexp) extends Rexp {
+  override def simp = (r1.simp, r2.simp) match {
+    case (NULL, _) => NULL
+    case (_, NULL) => NULL
+    case (EMPTY, r) => r
+    case (r, EMPTY) => r
+    case (r1, r2) => SEQ(r1, r2)
+  }
+}
+case class STAR(r: Rexp) extends Rexp {
+  override def simp = r.simp match {
+    case NULL => EMPTY
+    case EMPTY => EMPTY
+    case r => STAR(r)
+  }
+}
+
+// some convenience for typing in regular expressions
+def charlist2rexp(s : List[Char]) : Rexp = s match {
+  case Nil => EMPTY
+  case c::Nil => CHAR(c)
+  case c::s => SEQ(CHAR(c), charlist2rexp(s))
+}
+implicit def string2rexp(s : String) : Rexp = charlist2rexp(s.toList)
+
+
+// nullable function: tests whether the regular 
+// expression can recognise the empty string
+def nullable (r: Rexp) : Boolean = r match {
+  case NULL => false
+  case EMPTY => true
+  case CHAR(_) => false
+  case ALT(r1, r2) => nullable(r1) || nullable(r2)
+  case SEQ(r1, r2) => nullable(r1) && nullable(r2)
+  case STAR(_) => true
+}
+
+// derivative of a regular expression w.r.t. a character
+def der (c: Char, r: Rexp) : Rexp = r match {
+  case NULL => NULL
+  case EMPTY => NULL
+  case CHAR(d) => if (c == d) EMPTY else NULL
+  case ALT(r1, r2) => ALT(der(c, r1), der(c, r2))
+  case SEQ(r1, r2) => 
+    if (nullable(r1)) ALT(SEQ(der(c, r1), r2), der(c, r2))
+    else SEQ(der(c, r1), r2)
+  case STAR(r) => SEQ(der(c, r), STAR(r))
+}
+
+// derivative w.r.t. a string (iterates der)
+def ders (s: List[Char], r: Rexp) : Rexp = s match {
+  case Nil => r
+  case c::s => ders(s, der(c, r).simp)
+}
+
+// main matcher function
+def matcher(r: Rexp, s: String) : Boolean = nullable(ders(s.toList, r))
+
+
+//one or zero
+def OPT(r: Rexp) = ALT(r, EMPTY)
+
+//n-times
+def NTIMES(r: Rexp, n: Int) : Rexp = n match {
+  case 0 => EMPTY
+  case 1 => r
+  case n => SEQ(r, NTIMES(r, n - 1))
+}
+
+def RTEST(n: Int) = SEQ(NTIMES(OPT("a"), n), NTIMES("a", n))
+
+def time_needed[T](i: Int, code: => T) = {
+  val start = System.nanoTime()
+  for (j <- 1 to i) code
+  val end = System.nanoTime()
+  (end - start)/(i * 1.0e9)
+}
+
+for (i <- 1 to 100) {
+  println(i + ": " + "%.5f".format(time_needed(1, matcher(RTEST(i), "a" * i))))
+}
+
+