diff --git a/src/core/CCString.cppo.ml b/src/core/CCString.cppo.ml
index 246811bf..6a06e672 100644
--- a/src/core/CCString.cppo.ml
+++ b/src/core/CCString.cppo.ml
@@ -87,6 +87,30 @@ let is_sub ~sub i s j ~len =
   if i+len > String.length sub then invalid_arg "String.is_sub";
   _is_sub ~sub i s j ~len
 
+(* note: inefficient *)
+let find ?(start=0) ~sub s =
+  let n = String.length sub in
+  let i = ref start in
+  try
+    while !i + n < String.length s do
+      if _is_sub ~sub 0 s !i ~len:n then raise Exit;
+      incr i
+    done;
+    -1
+  with Exit ->
+    !i
+
+let rfind ~sub s =
+  let n = String.length sub in
+  let i = ref (String.length s - n) in
+  try
+    while !i >= 0 do
+      if _is_sub ~sub 0 s !i ~len:n then raise Exit;
+      decr i
+    done;
+    ~-1
+  with Exit ->
+    !i
 
 module Split = struct
   type split_state =
@@ -158,20 +182,17 @@ module Split = struct
 
   let seq ~by s = _mkseq ~by s _tuple3
   let seq_cpy ~by s = _mkseq ~by s String.sub
-end
 
-(* note: inefficient *)
-let find ?(start=0) ~sub s =
-  let n = String.length sub in
-  let i = ref start in
-  try
-    while !i + n < String.length s do
-      if _is_sub ~sub 0 s !i ~len:n then raise Exit;
-      incr i
-    done;
-    -1
-  with Exit ->
-    !i
+  let left ~by s =
+    let i = find ~sub:by s in
+    if i = ~-1 then None
+    else Some (String.sub s 0 i, String.sub s (i+1) (String.length s - i - 1))
+
+  let right ~by s =
+    let i = rfind ~sub:by s in
+    if i = ~-1 then None
+    else Some (String.sub s 0 i, String.sub s (i+1) (String.length s - i - 1))
+end
 
 let repeat s n =
   assert (n>=0);
@@ -252,11 +273,6 @@ let of_list l =
   List.iter (Buffer.add_char buf) l;
   Buffer.contents buf
 
-(*$T
-  of_list ['a'; 'b'; 'c'] = "abc"
-  of_list [] = ""
-*)
-
 let of_array a =
   init (Array.length a) (fun i -> a.(i))
 
@@ -285,11 +301,80 @@ let set s i c =
   if i<0 || i>= String.length s then invalid_arg "CCString.set";
   init (String.length s) (fun j -> if i=j then c else s.[j])
 
-(*$T
-  set "abcd" 1 '_' = "a_cd"
-  set "abcd" 0 '-' = "-bcd"
-  (try set "abc" 5 '_'; false with Invalid_argument _ -> true)
-*)
+let iter = String.iter
+
+#if OCAML_MAJOR >= 4
+
+let map = String.map
+let mapi = String.mapi
+let iteri = String.iteri
+
+#else
+
+let map f s = init (length s) (fun i -> f s.[i])
+let mapi f s = init (length s) (fun i -> f i s.[i])
+
+let iteri f s =
+  for i = 0 to String.length s - 1 do
+    f i s.[i]
+  done
+
+#endif
+
+let flat_map ?sep f s =
+  let buf = Buffer.create (String.length s) in
+  iteri
+    (fun i c ->
+       begin match sep with
+       | Some _ when i=0 -> ()
+       | None -> ()
+       | Some sep -> Buffer.add_string buf sep
+       end;
+       Buffer.add_string buf (f c)
+    ) s;
+  Buffer.contents buf
+
+exception MyExit
+
+let for_all p s =
+  try iter (fun c -> if not (p c) then raise MyExit) s; true
+  with MyExit -> false
+
+let exists p s =
+  try iter (fun c -> if p c then raise MyExit) s; false
+  with MyExit -> true
+
+let map2 f s1 s2 =
+  if length s1 <> length s2 then invalid_arg "String.map2";
+  init (String.length s1) (fun i -> f s1.[i] s2.[i])
+
+let iter2 f s1 s2 =
+  if length s1 <> length s2 then invalid_arg "String.iter2";
+  for i = 0 to String.length s1 - 1 do
+    f s1.[i] s2.[i]
+  done
+
+let iteri2 f s1 s2 =
+  if length s1 <> length s2 then invalid_arg "String.iteri2";
+  for i = 0 to String.length s1 - 1 do
+    f i s1.[i] s2.[i]
+  done
+
+let fold2 f acc s1 s2 =
+  if length s1 <> length s2 then invalid_arg "String.fold2";
+  let rec fold' acc s1 s2 i =
+    if i = String.length s1 then acc
+    else fold' (f acc s1.[i] s2.[i]) s1 s2 (i+1)
+  in
+  fold' acc s1 s2 0
+
+let for_all2 p s1 s2 =
+  try iter2 (fun c1 c2 -> if not (p c1 c2) then raise MyExit) s1 s2; true
+  with MyExit -> false
+
+let exists2 p s1 s2 =
+  try iter2 (fun c1 c2 -> if p c1 c2 then raise MyExit) s1 s2; false
+  with MyExit -> true
 
 let pp buf s =
   Buffer.add_char buf '"';
diff --git a/src/core/CCString.mli b/src/core/CCString.mli
index 50c7f417..65de38d7 100644
--- a/src/core/CCString.mli
+++ b/src/core/CCString.mli
@@ -81,12 +81,35 @@ val of_klist : char klist -> string
 val of_list : char list -> string
 val of_array : char array -> string
 
+(*$T
+  of_list ['a'; 'b'; 'c'] = "abc"
+  of_list [] = ""
+*)
+
 val to_array : string -> char array
 
 val find : ?start:int -> sub:string -> string -> int
 (** Find [sub] in string, returns its first index or [-1].
     Should only be used with very small [sub] *)
 
+(*$T
+  find ~sub:"bc" "abcd" = 1
+  find ~sub:"bc" "abd" = ~-1
+  find ~sub:"a" "_a_a_a_" = 1
+*)
+
+val rfind : sub:string -> string -> int
+(** Find [sub] in string from the right, returns its first index or [-1].
+    Should only be used with very small [sub]
+    @since NEXT_RELEASE *)
+
+(*$T
+  rfind ~sub:"bc" "abcd" = 1
+  rfind ~sub:"bc" "abd" = ~-1
+  rfind ~sub:"a" "_a_a_a_" = 5
+  rfind ~sub:"bc" "abcdbcd" = 4
+*)
+
 val is_sub : sub:string -> int -> string -> int -> len:int -> bool
 (** [is_sub ~sub i s j ~len] returns [true] iff the substring of
     [sub] starting at position [i] and of length [len] *)
@@ -143,8 +166,75 @@ val set : string -> int -> char -> string
     @raise Invalid_argument if [i] is an invalid index
     @since NEXT_RELEASE *)
 
+(*$T
+  set "abcd" 1 '_' = "a_cd"
+  set "abcd" 0 '-' = "-bcd"
+  (try ignore (set "abc" 5 '_'); false with Invalid_argument _ -> true)
+*)
+
+val iter : (char -> unit) -> string -> unit
+(** Alias to {!String.iter}
+    @since NEXT_RELEASE *)
+
+val iteri : (int -> char -> unit) -> string -> unit
+(** iter on chars with their index
+    @since NEXT_RELEASE *)
+
+val map : (char -> char) -> string -> string
+(** map chars
+    @since NEXT_RELEASE *)
+
+val mapi : (int -> char -> char) -> string -> string
+(** map chars with their index
+    @since NEXT_RELEASE *)
+
+val flat_map : ?sep:string -> (char -> string) -> string -> string
+(** map each chars to a string, then concatenates them all
+    @param sep optional separator between each generated string
+    @since NEXT_RELEASE *)
+
+val for_all : (char -> bool) -> string -> bool
+(** true for all chars?
+    @since NEXT_RELEASE *)
+
+val exists : (char -> bool) -> string -> bool
+(** true for some char?
+    @since NEXT_RELEASE *)
+
 include S with type t := string
 
+(** {2 Operations on 2 strings} *)
+
+val map2 : (char -> char -> char) -> string -> string -> string
+(** map pairs of chars
+    @raises Invalid_argument if the strings have not the same length
+    @since NEXT_RELEASE *)
+
+val iter2: (char -> char -> unit) -> string -> string -> unit
+(** iterate on pairs of chars
+    @raises Invalid_argument if the strings have not the same length
+    @since NEXT_RELEASE *)
+
+val iteri2: (int -> char -> char -> unit) -> string -> string -> unit
+(** iterate on pairs of chars with their index
+    @raises Invalid_argument if the strings have not the same length
+    @since NEXT_RELEASE *)
+
+val fold2: ('a -> char -> char -> 'a) -> 'a -> string -> string -> 'a
+(** fold on pairs of chars
+    @raises Invalid_argument if the strings have not the same length
+    @since NEXT_RELEASE *)
+
+val for_all2 : (char -> char -> bool) -> string -> string -> bool
+(** all pair of chars respect the predicate?
+    @raises Invalid_argument if the strings have not the same length
+    @since NEXT_RELEASE *)
+
+val exists2 : (char -> char -> bool) -> string -> string -> bool
+(** exists a pair of chars?
+    @raises Invalid_argument if the strings have not the same length
+    @since NEXT_RELEASE *)
+
 (** {2 Splitting} *)
 
 module Split : sig
@@ -181,6 +271,26 @@ module Split : sig
   val seq_cpy : by:string -> string -> string sequence
 
   val klist_cpy : by:string -> string -> string klist
+
+  val left : by:string -> string -> (string * string) option
+  (** Split on the first occurrence of [by] from the left-most part of
+      the string
+      @since NEXT_RELEASE *)
+
+  (*$T
+    Split.left ~by:" " "ab cde f g " = Some ("ab", "cde f g ")
+    Split.left ~by:"_" "abcde" = None
+  *)
+
+  val right : by:string -> string -> (string * string) option
+  (** Split on the first occurrence of [by] from the rightmost part of
+      the string
+      @since NEXT_RELEASE *)
+
+  (*$T
+    Split.right ~by:" " "ab cde f g" = Some ("ab cde f", "g")
+    Split.right ~by:"_" "abcde" = None
+  *)
 end
 
 (** {2 Slices} A contiguous part of a string *)