diff -r e85600529ca5 -r 4794759139ea progs/regexp.scala
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/progs/regexp.scala	Sat Jun 15 09:23:18 2013 -0400
@@ -0,0 +1,106 @@
+// regular expressions
+abstract class Rexp
+
+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
+case class SEQ(r1: Rexp, r2: Rexp) extends Rexp
+case class STAR(r: Rexp) extends Rexp
+
+// 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)
+
+
+// for example
+println(STAR("abc"))
+
+// produces STAR(SEQ(CHAR(a),SEQ(CHAR(b),SEQ(CHAR(c),EMPTY))))
+
+
+
+// a simple-minded regular expression matcher:
+// it loops for examples like STAR(EMPTY) with
+// strings this regular expression does not match
+
+def smatchers(rs: List[Rexp], s: List[Char]) : Boolean = (rs, s) match {
+  case (NULL::rs, s) => false
+  case (EMPTY::rs, s) => smatchers(rs, s)
+  case (CHAR(c)::rs, Nil) => false
+  case (CHAR(c)::rs, d::s) => (c ==d) && smatchers(rs, s) 
+  case (ALT(r1, r2)::rs, s) => smatchers(r1::rs, s) || smatchers(r2::rs, s)
+  case (SEQ(r1, r2)::rs, s) => smatchers(r1::r2::rs, s) 
+  case (STAR(r)::rs, s) => smatchers(rs, s) || smatchers(r::STAR(r)::rs, s) 
+  case (Nil, s) => s == Nil
+}
+
+def smatcher(r: Rexp, s: String) = smatchers(List(r), s.toList)
+
+// regular expression: a
+println(smatcher(CHAR('a'), "ab")) 
+
+// regular expression: a + (b o c)
+println(smatcher(ALT(CHAR('a'), SEQ(CHAR('b'), CHAR('c'))), "ab")) 
+
+// regular expression: a + (b o c)
+println(smatcher(ALT(CHAR('a'), SEQ(CHAR('b'), CHAR('c'))), "bc")) 
+
+// loops for regular expression epsilon* 
+//println(smatcher(STAR(EMPTY), "a"))
+
+
+
+// Regular expression matcher that works properly
+//================================================
+
+// 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))
+}
+
+// main matcher function
+def matcher(r: Rexp, s: String) : Boolean = nullable(ders(s.toList, r))
+
+
+//examples
+
+println(matcher(SEQ(STAR("a"), STAR("b")), "bbaaa"))
+println(matcher(ALT(STAR("a"), STAR("b")), ""))
+println(matcher("abc", ""))
+println(matcher(STAR(ALT(EMPTY, "a")), ""))
+println(matcher(STAR(EMPTY), "a"))
+println(matcher("cab","cab"))
+println(matcher(STAR("a"),"aaa"))
+println(matcher("cab" ,"cab"))
+println(matcher(STAR("a"),"aaa"))
+
+