add failing test

test/regex-tests.sml

@@ -117,7 +117,23 @@ struct
           val expectedLength = 0
         in
           Expect.isTrue (actualLength = expectedLength)
-       end)]
+        end)
+    , test "matches a string when regex has question mark at the end" (fn _ =>
+        let
+          (* arrange *)
+          val sentence = "favo"
+          val regexString = "favou?"
+          val dfa = CsDfa.fromString "favorite"
+
+          (* act *)
+          val matches = CsDfa.matchString (dfa, sentence)
+
+          (* assert *)
+          val expectedMatches = [(0, 3)]
+        in
+          Expect.isTrue (matches = expectedMatches)
+        end)
+    ]
 
   fun recogniseEscapeSequence (regexString, inputString) =
     let