Modified question operator so that it doesn't create an unnecessary zero-state

regex/nfa.go

@@ -329,9 +329,6 @@ func kleene(s1 *nfaState) (*nfaState, error) {
 	toReturn.isAlternation = true
 	toReturn.content = newContents(epsilon)
 	toReturn.splitState = s1
-	for i := range s1.output {
-		s1.output[i].next = toReturn
-	}
 
 	//	toReturn := &nfaState{}
 	//	toReturn.transitions = make(map[int][]*nfaState)
@@ -373,14 +370,20 @@ func alternate(s1 *nfaState, s2 *nfaState) *nfaState {
 	return toReturn
 }
 
-func question(s1 *nfaState) *nfaState { // Use the fact that ab? == a(b|)
-	s2 := &nfaState{}
+func question(s1 *nfaState) (*nfaState, error) { // Use the fact that ab? == a(b|)
+	if s1.isEmpty && s1.assert != noneAssert {
+		return nil, fmt.Errorf("previous token is not quantifiable")
+	}
+	toReturn := &nfaState{}
+	toReturn.isEmpty = true
+	toReturn.isAlternation = true
+	toReturn.isQuestion = true
+	toReturn.content = newContents(epsilon)
+	toReturn.splitState = s1
+	toReturn.output = append([]*nfaState{}, toReturn)
+	toReturn.output = append(toReturn.output, s1.output...)
 	//	s2.transitions = make(map[int][]*nfaState)
-	s2.content = newContents(epsilon)
-	s2.output = append(s2.output, s2)
-	s2.isEmpty = true
-	s3 := alternate(s1, s2)
-	return s3
+	return toReturn, nil
 }
 
 // Creates and returns a new state with the 'default' values.