diff --git a/demo-repository/exercises/demo2/meta.json b/demo-repository/exercises/demo2/meta.json
index 6f1d64e8b..bcde52b7e 100644
--- a/demo-repository/exercises/demo2/meta.json
+++ b/demo-repository/exercises/demo2/meta.json
@@ -1,2 +1,2 @@
-{"learnocaml_version":"1","kind":"exercise","stars":0,
+{"learnocaml_version":"1","kind":"exercise","stars":1,
  "title":"Demo of the exercise environment (MD version)"}
diff --git a/src/app/dune b/src/app/dune
index addb087bb..c0230bcc1 100644
--- a/src/app/dune
+++ b/src/app/dune
@@ -25,9 +25,19 @@
     learnocaml_data
     learnocaml_api
     sha
+    re.pcre
     ocplib_i18n)
 )
 
+(library
+ (name learnocaml_app_common_test)
+ (wrapped false)
+ (modules Learnocaml_app_common_test)
+ (libraries learnocaml_app_common)
+ (inline_tests)
+ (preprocess (pps ppx_expect ppx_inline_test))
+)
+
 (executable
  (name learnocaml_index_main)
  (modes byte js)
@@ -35,6 +45,7 @@
  (libraries ezjsonm
             ace
 	    sha
+	    re.pcre
             learnocaml_repository
             learnocaml_app_common
             learnocaml_toplevel
@@ -141,5 +152,4 @@
         (learnocaml_description_main.bc.js as www/js/learnocaml-description.js)
         (learnocaml_partition_view.bc.js as www/js/learnocaml-partition-view.js)
         (learnocaml_playground_main.bc.js as www/js/learnocaml-playground.js))
-)
-
+)
\ No newline at end of file
diff --git a/src/app/learnocaml_app_common_test.ml b/src/app/learnocaml_app_common_test.ml
new file mode 100644
index 000000000..9a4075545
--- /dev/null
+++ b/src/app/learnocaml_app_common_test.ml
@@ -0,0 +1,112 @@
+open Learnocaml_common
+
+
+let to_test_encode s = Printf.printf "%s " (encode s)
+let to_test_decode s = Printf.printf "%s " (decode s)
+let to_test_bijection1 s = Printf.printf "%s " (decode (encode s))
+let to_test_bijection2 s = Printf.printf "%s " (encode (decode s))
+
+let original_e  =  ["=test";"test=test";"test=";"==";""]
+let coded_e = ["-etest";"test-etest";"test-e";"-e-e";""]
+
+let original_c  =  [",test";"test,test";"test,";",,";""]
+let coded_c = ["-ctest";"test-ctest";"test-c";"-c-c";""]
+
+let original_a  =  ["&test";"test&test";"test&";"&&";""]
+let coded_a = ["-atest";"test-atest";"test-a";"-a-a";""]
+
+let original__  =  ["-test";"test-test";"test-";"--";""]
+let coded__ = ["--test";"test--test";"test--";"----";""]
+
+let original_all = ["=,&-";"=,-&";"=&,-";"=&-,";"=-,&";"=-&,";
+                    ",=&-";",=-&";",&=-";",&-=";",-=&";",-&=";
+                    "&=,-";"&=-,";"&,=-";"&,-=";"&-=,";"&-,=";
+                    "-=,&";"-=&,";"-,=&";"-,&=";"-&=,";"-&,="]
+let codedall = ["-e-c-a--";"-e-c---a";"-e-a-c--";"-e-a---c";"-e---c-a";"-e---a-c"; 
+                "-c-e-a--";"-c-e---a";"-c-a-e--";"-c-a---e";"-c---e-a";"-c---a-e"; 
+                "-a-e-c--";"-a-e---c";"-a-c-e--";"-a-c---e";"-a---e-c";"-a---c-e";
+                "---e-c-a";"---e-a-c";"---c-a-e";"---c-a-e";"---a-e-c";"---a-c-e"]
+
+
+let%expect_test "encode -e" = 
+  List.iter to_test_encode original_e;
+  [%expect {|-etest test-etest test-e -e-e  |}]
+
+let%expect_test "encode -c" = 
+  List.iter to_test_encode original_c;
+  [%expect {|-ctest test-ctest test-c -c-c  |}]
+
+let%expect_test "encode -a" = 
+  List.iter to_test_encode original_a;
+  [%expect {|-atest test-atest test-a -a-a  |}]
+
+let%expect_test "encode --" = 
+  List.iter to_test_encode original__;
+  [%expect {|--test test--test test-- ----  |}]
+
+let%expect_test "encode all" =
+  List.iter to_test_encode original_all;
+  [%expect {|-e-c-a-- -e-c---a -e-a-c-- -e-a---c -e---c-a -e---a-c -c-e-a-- -c-e---a -c-a-e-- -c-a---e -c---e-a -c---a-e -a-e-c-- -a-e---c -a-c-e-- -a-c---e -a---e-c -a---c-e ---e-c-a ---e-a-c ---c-a-e ---c-a-e ---a-e-c ---a-c-e |}]
+
+
+let%expect_test "decode -e" = 
+  List.iter to_test_decode coded_e;
+  [%expect {|=test test=test test= ==  |}]
+
+let%expect_test "decode -c" = 
+  List.iter to_test_decode coded_c;
+  [%expect {|,test test,test test, ,,  |}]
+
+let%expect_test "decode -a" = 
+  List.iter to_test_decode coded_a;
+  [%expect {|&test test&test test& &&  |}]
+
+let%expect_test "decode --" = 
+  List.iter to_test_decode coded_e;
+  [%expect {|-test test-test test- --  |}]
+
+let%expect_test "decode all" =
+  List.iter to_test_decode codedall;
+  [%expect {|=,&- =,-& =&,- =&-, =-,& =-&, ,=&- ,=-& ,&=- ,&-= ,-=& ,-&= &=,- &=-, &,=- &,-= &-=, &-,= -=,& -=&, -,=& -,&= -&=, -&,= |}]
+
+
+let%expect_test "bijection 1 -e" = 
+  List.iter to_test_bijection1 original_e;
+  [%expect {|=test test=test test= ==  |}]
+
+let%expect_test "bijection 1 -c" = 
+  List.iter to_test_bijection1 original_c;
+  [%expect {|,test test,test test, ,,  |}]
+
+let%expect_test "bijection 1 -a" = 
+  List.iter to_test_bijection1 original_a;
+  [%expect {|&test test&test test& &&  |}]
+
+let%expect_test "bijection 1 --" = 
+  List.iter to_test_bijection1 original__;
+  [%expect {|-test test-test test- --  |}]
+
+let%expect_test "bijection 1 all" = 
+  List.iter to_test_bijection1 original_all;
+  [%expect {|=,&- =,-& =&,- =&-, =-,& =-&, ,=&- ,=-& ,&=- ,&-= ,-=& ,-&= &=,- &=-, &,=- &,-= &-=, &-,= -=,& -=&, -,=& -,&= -&=, -&,= |}]
+
+
+let%expect_test "bijection 2 -e" = 
+  List.iter to_test_bijection2 coded_e;
+  [%expect {|-etest test-etest test-e -e-e  |}]
+
+let%expect_test "bijection 2 -c" = 
+  List.iter to_test_bijection2 coded_c;
+  [%expect {|-ctest test-ctest test-c -c-c  |}]
+
+let%expect_test "bijection 2 -a" =
+  List.iter to_test_bijection2 coded_a;
+  [%expect {|-atest test-atest test-a -a-a  |}]
+
+let%expect_test "bijection 2 --" = 
+  List.iter to_test_bijection2 coded__;
+  [%expect {|--test test--test test-- ----  |}]
+
+let%expect_test "bijection 2 all" = 
+  List.iter to_test_bijection2 codedall;
+  [%expect {|-e-c-a-- -e-c---a -e-a-c-- -e-a---c -e---c-a -e---a-c -c-e-a-- -c-e---a -c-a-e-- -c-a---e -c---e-a -c---a-e -a-e-c-- -a-e---c -a-c-e-- -a-c---e -a---e-c -a---c-e ---e-c-a ---e-a-c ---c-a-e ---c-a-e ---a-e-c ---a-c-e |}]
\ No newline at end of file
diff --git a/src/app/learnocaml_common.ml b/src/app/learnocaml_common.ml
index a5a12d2d6..b87f08276 100644
--- a/src/app/learnocaml_common.ml
+++ b/src/app/learnocaml_common.ml
@@ -13,9 +13,21 @@ open Js_utils
 open Lwt.Infix
 open Learnocaml_data
 open Learnocaml_config
+open Re.Pcre
 
 module H = Tyxml_js.Html
 
+let encode str =
+  Re.Pcre.substitute ~rex:(Re.Pcre.regexp ",") ~subst:(fun _ -> "-c") (
+      Re.Pcre.substitute ~rex:(Re.Pcre.regexp "&") ~subst:(fun _ -> "-a") (
+          Re.Pcre.substitute ~rex:(Re.Pcre.regexp "=") ~subst:(fun _ -> "-e") (
+              Re.Pcre.substitute ~rex:(Re.Pcre.regexp "-") ~subst:(fun _ -> "--") str)))
+let decode str =
+  Re.Pcre.substitute ~rex:(Re.Pcre.regexp "--") ~subst:(fun _ -> "-") (
+      Re.Pcre.substitute ~rex:(Re.Pcre.regexp "-e") ~subst:(fun _ -> "=") (
+          Re.Pcre.substitute ~rex:(Re.Pcre.regexp "-a") ~subst:(fun _ -> "&") (
+              Re.Pcre.substitute ~rex:(Re.Pcre.regexp "-c") ~subst:(fun _ -> ",") str)))
+
 let find_div_or_append_to_body id =
   match Manip.by_id id with
   | Some div -> div
diff --git a/src/app/learnocaml_common.mli b/src/app/learnocaml_common.mli
index eff755d06..1d0f8f8ae 100644
--- a/src/app/learnocaml_common.mli
+++ b/src/app/learnocaml_common.mli
@@ -10,6 +10,10 @@ open Js_of_ocaml
 open Js_of_ocaml_tyxml
 open Learnocaml_data
 
+val encode : string -> string
+
+val decode : string -> string
+
 val find_div_or_append_to_body : string -> [> Html_types.div ] Tyxml_js.Html.elt
 
 val find_component : string -> 'a Tyxml_js.Html.elt
diff --git a/src/app/learnocaml_index_main.ml b/src/app/learnocaml_index_main.ml
index 29a3d9dc6..8f5160e51 100644
--- a/src/app/learnocaml_index_main.ml
+++ b/src/app/learnocaml_index_main.ml
@@ -10,11 +10,13 @@ open Js_of_ocaml
 open Js_of_ocaml_tyxml
 open Js_of_ocaml_lwt
 open Js_utils
+open Re.Pcre
 open Lwt
 open Learnocaml_data
 open Learnocaml_common
 open Learnocaml_config
 
+
 module H = Tyxml_js.Html5
 
 module El = struct
@@ -85,8 +87,85 @@ let (exercise_filter_signal: string option React.signal), set_exercise_filter =
 let (exercise_sort_signal: exercise_ordering React.signal), set_exercise_sort =
   React.S.create By_category
 
+
+let (expand_state_signal: string list React.signal), set_expand_state =
+  React.S.create []
+
+let join_ids l =
+  match l with
+  | [] -> ""
+  | h::t -> List.fold_left (fun acc x -> acc ^ "," ^ x) h t
+
+let split_ids str =
+  Re.Pcre.split ~rex:(Re.Pcre.regexp ",") str
+
+let encode str =
+  let open Re.Pcre in
+  substitute ~rex:(regexp ",") ~subst:(fun _ -> "-c")
+  @@ substitute ~rex:(regexp "&") ~subst:(fun _ -> "-a")
+  @@ substitute ~rex:(regexp "=") ~subst:(fun _ -> "-e")
+  @@ substitute ~rex:(regexp "-") ~subst:(fun _ -> "--")
+  @@ str
+
+let decode str =
+  let open Re.Pcre in
+  substitute ~rex:(regexp "--") ~subst:(fun _ -> "-")
+  @@ substitute ~rex:(regexp "-e") ~subst:(fun _ -> "=")
+  @@ substitute ~rex:(regexp "-a") ~subst:(fun _ -> "&")
+  @@ substitute ~rex:(regexp "-c") ~subst:(fun _ -> ",")
+  @@ str
+
+(* Update the expand list in fragment to fit the URL *)
+let rec update_expand ?value fragment =
+  match value with
+  | Some v ->
+     if List.mem_assoc "expand" fragment then
+       match fragment with
+       | [] -> []
+       | ("expand", x)::t ->
+          let expand_list = split_ids x in
+          let new_expand_list =
+            if List.mem v expand_list then
+              begin
+                let filtered_list = List.filter (fun x -> x <> v) expand_list in
+                if filtered_list = [] then [""] else filtered_list
+              end
+            else
+              begin
+                if expand_list = [""] then [v] else v :: expand_list
+              end
+          in
+          let joined_expand = join_ids new_expand_list in
+          set_expand_state new_expand_list;
+          ("expand", joined_expand)::t
+       | h::t ->
+          h::(update_expand ~value:v t)
+     else
+       begin
+         set_expand_state [v];
+         fragment @ [("expand", v)]
+       end
+  | None ->
+     set_expand_state [];
+     List.filter (fun (k, _) -> k <> "expand") fragment
+
+let update_sort value fragment =
+  let filtered_fragment = List.remove_assoc "sort" (update_expand fragment) in
+  filtered_fragment @ [("sort", value)] @ [("expand", "")]
+
+let update_fragment key value =
+  let fragment = Js_utils.parse_fragment () in
+  let filtered_fragment =
+    if key = "expand" then
+      let v = (Uri.pct_encode (encode value)) in
+      update_expand ~value:v fragment
+    else
+      update_sort value fragment
+  in
+  Js_utils.set_fragment filtered_fragment
+
 let make_exercises_to_display_signal index =
-  let get_index exo_sort exo_filter =
+  let get_index exo_sort exo_filter _expand =
     let index =
       match exo_sort with
       | By_category -> index
@@ -113,7 +192,7 @@ let make_exercises_to_display_signal index =
             StrMap.fold (fun skill exercises acc ->
                 (skill,
                  {Exercise.Index.title = skill;
-                  contents = Exercise.Index.Exercises (List.rev exercises)})
+                   contents = Exercise.Index.Exercises (List.rev exercises)})
                 :: acc)
               by_skill []
           in
@@ -165,7 +244,7 @@ let make_exercises_to_display_signal index =
            Exercise.Index.contents = Exercise.Index.Exercises []; }]
     else index
   in
-  React.S.l2 get_index exercise_sort_signal exercise_filter_signal
+  React.S.l3 get_index exercise_sort_signal exercise_filter_signal expand_state_signal
 
 let retain_signals = ref (React.S.const ())
 (* Used to register signals as GC roots *)
@@ -173,6 +252,19 @@ let retain_signals = ref (React.S.const ())
 let exercises_tab token : tab_handler =
   fun _ _ () ->
     let open Tyxml_js.Html5 in
+    let () =
+      Dom_html.window##.onhashchange := Dom_html.handler (fun _ ->
+        let fragment = Js_utils.parse_fragment () in
+        (match List.assoc_opt "sort"  fragment with
+        | Some "category" -> set_exercise_sort By_category
+        | Some "skill" -> set_exercise_sort By_skill
+        | Some "difficulty" -> set_exercise_sort By_difficulty
+        | _ -> ());
+        (match List.assoc_opt "expand" fragment with
+        | None -> set_expand_state []
+        | Some e -> set_expand_state (split_ids e));
+        Js._true)
+    in
     show_loading  [%i"Loading exercises"] @@ fun () ->
     Lwt_js.sleep 0.5 >>= fun () ->
     retrieve (Learnocaml_api.Exercise_index token)
@@ -236,6 +328,12 @@ let exercises_tab token : tab_handler =
           ]
         ] ]
     in
+    let update_expand_class id ids =
+      if List.mem id ids then
+        []
+      else
+        ["collapsed"]
+    in
     let rec format_exercise_list index =
       match index with
       | Exercise.Index.Exercises el ->
@@ -248,7 +346,25 @@ let exercises_tab token : tab_handler =
           List.map (fun (id, grp) ->
               let clas =
                 "group-title" ::
-                match gl with [] | [_] -> [] | _ -> ["collapsed"]
+                  match gl with
+                  | [] -> []
+                  | [_] ->
+                     let expand_ids = React.S.value expand_state_signal in
+                     if expand_ids = [] then
+                       begin
+                         let encoded = encode id in
+                         set_expand_state [(encoded)];
+                         update_fragment "expand" (encoded);
+                         []
+                       end
+                     else
+                       update_expand_class (encode id) expand_ids
+                  | _ ->
+                     let expand_ids = React.S.value expand_state_signal in
+                     if expand_ids = [] then
+                       ["collapsed"]
+                     else
+                       update_expand_class (encode id) expand_ids
               in
               let title =
                 H.div ~a:[a_id id; a_class clas]
@@ -257,8 +373,9 @@ let exercises_tab token : tab_handler =
               let exos = format_exercise_list grp.Exercise.Index.contents in
               Manip.Ev.onclick title
                 (fun _ ->
-                   ignore (Manip.toggleClass title "collapsed");
-                   false);
+                  update_fragment "expand" id;
+                  ignore (Manip.toggleClass title "collapsed");
+                  false);
               H.li [title; exos])
             gl
     in
@@ -269,7 +386,14 @@ let exercises_tab token : tab_handler =
       List.map (fun (id, sort, name) ->
           let btn = button ~a:[a_id id] [ txt name ] in
           Manip.Ev.onclick btn
-            (fun _ -> set_exercise_sort sort; true);
+            (fun _ ->
+              let sort_value =
+                match sort with
+                | By_category -> "category"
+                | By_skill -> "skill"
+                | By_difficulty -> "difficulty"
+              in
+              update_fragment "sort" sort_value; set_exercise_sort sort; true);
           let signal =
             React.S.map (fun s ->
                 (if sort = s then Manip.addClass else Manip.removeClass)
@@ -311,6 +435,17 @@ let exercises_tab token : tab_handler =
     in
     retain_signals :=
       React.S.merge (fun () () -> ()) () (list_update_signal :: btns_sigs);
+    let () =
+      ignore (React.S.map (fun expanded_ids ->
+        List.iter (fun id ->
+          match Dom_html.getElementById_opt id with
+          | Some elt ->
+            let class_list = elt##.classList in
+            if Js.to_bool (class_list##contains (Js.string "group-title")) &&
+               Js.to_bool (class_list##contains (Js.string "collapsed"))
+            then ignore (class_list##remove (Js.string "collapsed"))
+          | None -> ()) expanded_ids) expand_state_signal)
+    in
     Lwt.return pane_div
 
 let playground_tab token : tab_handler =
diff --git a/src/state/learnocaml_common_test.ml b/src/state/learnocaml_common_test.ml
new file mode 100644
index 000000000..32931042b
--- /dev/null
+++ b/src/state/learnocaml_common_test.ml
@@ -0,0 +1,57 @@
+open Learnocaml_common
+
+let pred_bijection s = decode (encode s) = s
+let print_encode s = Printf.printf "%s " (encode s)
+
+
+let testcase_eq = ["=test";"test=test";"test=";"==";""]
+
+let%expect_test "encode '=' as '-e'" = 
+  List.iter print_encode testcase_eq;
+  [%expect_exact {|-etest test-etest test-e -e-e  |}]
+
+let%test "decode @@ encode == id" = 
+  List.for_all pred_bijection testcase_eq
+
+
+let testcase_co = [",test"; "test,test"; "test,"; ",,"; ""]
+
+let%expect_test "encode ',' as '-c'" = 
+  List.iter print_encode testcase_co;
+  [%expect_exact {|-ctest test-ctest test-c -c-c  |}]
+
+let%test "decode @@ encode == id" = 
+  List.for_all pred_bijection testcase_co
+
+
+let testcase_an = ["&test"; "test&test"; "test&"; "&&"; ""]
+
+let%expect_test "encode '&' as '-a'" = 
+  List.iter print_encode testcase_an;
+  [%expect_exact {|-atest test-atest test-a -a-a  |}]
+
+let%test "decode @@ encode == id" = 
+  List.for_all pred_bijection testcase_an
+
+
+let testcase_da = ["-test"; "test-test"; "test-"; "--"; ""]
+
+let%expect_test "encode '-' as '--'" = 
+  List.iter print_encode testcase_co;
+  [%expect_exact {|--test test--test test-- ----  |}]
+
+let%test "decode @@ encode == id" = 
+  List.for_all pred_bijection testcase_da
+
+
+let testcase_all = ["=,&-";"=,-&";"=&,-";"=&-,";"=-,&";"=-&,";
+                    ",=&-";",=-&";",&=-";",&-=";",-=&";",-&=";
+                    "&=,-";"&=-,";"&,=-";"&,-=";"&-=,";"&-,=";
+                    "-=,&";"-=&,";"-,=&";"-,&=";"-&=,";"-&,="]
+
+let%expect_test "encode all possibilities" = 
+  List.iter print_encode testcase_co;
+  [%expect_exact {|-e-c-a-- -e-c---a -e-a-c-- -e-a---c -e---c-a -e---a-c -c-e-a-- -c-e---a -c-a-e-- -c-a---e -c---e-a -c---a-e -a-e-c-- -a-e---c -a-c-e-- -a-c---e -a---e-c -a---c-e ---e-c-a ---e-a-c ---c-a-e ---c-a-e ---a-e-c ---a-c-e |}]
+
+let%test "decode @@ encode == id" = 
+  List.for_all pred_bijection testcase_all
\ No newline at end of file