initial viable version

rx.py

@@ -2,8 +2,14 @@ class Node(object):
     def derive(self, char):
         return NeverMatches
 
+    def match_end(self): return False
+    def can_match_more(self): return not self.match_end()
 
-EmptyString  = Node()
+
+class EmptyString(Node):
+    def match_end(self): return True
+
+EmptyString = EmptyString()
 NeverMatches = Node()
 
 
@@ -35,6 +41,12 @@ class AlternationNode(Node):
     def derive(self, char):
         return new_alternation([alt.derive(char) for alt in self.alts])
 
+    def match_end(self):
+        return any(alt.match_end() for alt in self.alts)
+
+    def can_match_more(self):
+        return any(alt.can_match_more() for alt in self.alts)
+
 
 class AnyNode(Node):
     def __init__(self, nxt):
@@ -52,6 +64,9 @@ class RepetitionNode(Node):
     def derive(self, char):
         return new_alternation([self.head.derive(char), self.nxt.derive(char)])
 
+    def match_end(self): return self.nxt.match_end()
+    def can_match_more(self): return True
+
 
 class Or:
     def __init__(self, alts):
@@ -134,7 +149,7 @@ class RE:
         for i, char in enumerate(s):
             state = state.derive(char)
 
-            if state is EmptyString: return i == ls-1
+            if state.match_end() and i == ls-1: return True
-            if state is NeverMatches: return False
+            if state.match_end() and not state.can_match_more : return False
 
         return False