diff --git a/README.md b/README.md
index 43610d5b..377b6852 100644
--- a/README.md
+++ b/README.md
@@ -60,6 +60,7 @@ structures comprise (some modules in `misc/`, some other in `core/`):
 - `CCKTree`, an abstract lazy tree structure (similar to what `CCKlist` is to lists)
 - small modules (basic types, utilities):
   - `CCInt`
+  - `CCString` (basic string operations)
   - `CCPair` (cartesian products)
   - `CCOpt` (options)
   - `CCFun` (function combinators)
@@ -70,6 +71,12 @@ structures comprise (some modules in `misc/`, some other in `core/`):
   - `CCHash` (hashing combinators)
   - `CCError` (monadic error handling)
 
+### String
+
+In the module `Containers_string`:
+- `Levenshtein`: edition distance between two strings
+- `KMP`: Knuth-Morris-Pratt substring algorithm
+
 ### Misc
 
 - `PHashtbl`, a polymorphic hashtable (with open addressing)
diff --git a/_oasis b/_oasis
index 5f35da1d..df6756cb 100644
--- a/_oasis
+++ b/_oasis
@@ -47,13 +47,13 @@ Library "containers"
                     CCMultiSet, CCBV, CCPrint, CCPersistentHashtbl, CCError,
                     CCHeap, CCList, CCOpt, CCPair, CCFun, CCHash,
                     CCKList, CCInt, CCBool, CCArray, CCBatch, CCOrd,
-                    CCRandom, CCLinq, CCKTree, CCTrie
+                    CCRandom, CCLinq, CCKTree, CCTrie, CCString
   FindlibName:      containers
 
 Library "containers_string"
   Path:             string
   Pack:             true
-  Modules:          KMP, CCString, Levenshtein
+  Modules:          KMP, Levenshtein
   FindlibName:      string
   FindlibParent:    containers
 
diff --git a/core/CCString.ml b/core/CCString.ml
new file mode 100644
index 00000000..ff37b99d
--- /dev/null
+++ b/core/CCString.ml
@@ -0,0 +1,265 @@
+
+(*
+copyright (c) 2013-2014, simon cruanes
+all rights reserved.
+
+redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+redistributions of source code must retain the above copyright notice, this
+list of conditions and the following disclaimer.  redistributions in binary
+form must reproduce the above copyright notice, this list of conditions and the
+following disclaimer in the documentation and/or other materials provided with
+the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*)
+
+(** {1 Basic String Utils} *)
+
+type 'a gen = unit -> 'a option
+type 'a sequence = ('a -> unit) -> unit
+type 'a klist = unit -> [`Nil | `Cons of 'a * 'a klist]
+
+module type S = sig
+  type t
+
+  val length : t -> int
+
+  val blit : t -> int -> t -> int -> int -> unit
+  (** See {!String.blit} *)
+
+  (** {2 Conversions} *)
+
+  val to_gen : t -> char gen
+
+  val to_seq : t -> char sequence
+
+  val to_klist : t -> char klist
+
+  val pp : Buffer.t -> t -> unit
+end
+
+type t = string
+
+let equal a b = a=b
+
+let compare = String.compare
+
+let hash s = Hashtbl.hash s
+
+let length = String.length
+
+let _is_sub ~sub i s j ~len =
+  let rec check k =
+    if k = len
+      then true
+      else sub.[i + k] = s.[j+k] && check (k+1)
+  in
+  j+len <= String.length s && check 0
+
+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
+
+
+module Split = struct
+  type split_state =
+    | SplitStop
+    | SplitAt of int (* previous *)
+
+  (* [by_j... prefix of s_i...] ? *)
+  let rec _is_prefix ~by s i j =
+    j = String.length by
+    ||
+    ( i < String.length s &&
+      s.[i] = by.[j] &&
+      _is_prefix ~by s (i+1) (j+1)
+    )
+
+  let rec _split ~by s state = match state with
+    | SplitStop -> None
+    | SplitAt prev -> _split_search ~by s prev prev
+  and _split_search ~by s prev i =
+    if i >= String.length s
+      then Some (SplitStop, prev, String.length s - prev)
+      else if _is_prefix ~by s i 0 && i>prev
+        then Some (SplitAt (i+String.length by), prev, i-prev)
+      else _split_search ~by s prev (i+1)
+
+  let _tuple3 x y z = x,y,z
+
+  let _mkgen ~by s k =
+    let state = ref (SplitAt 0) in
+    fun () ->
+      match _split ~by s !state with
+        | None -> None
+        | Some (state', i, len) ->
+            state := state';
+            Some (k s i len)
+
+  let gen ~by s = _mkgen ~by s _tuple3
+
+  let gen_cpy ~by s = _mkgen ~by s String.sub
+
+  let _mklist ~by s k =
+    let rec build acc state = match _split ~by s state with
+      | None -> List.rev acc
+      | Some (state', i, len) ->
+          build (k s i len ::acc) state'
+    in
+    build [] (SplitAt 0)
+
+  let list_ ~by s = _mklist ~by s _tuple3
+
+  let list_cpy ~by s = _mklist ~by s String.sub
+
+  let _mkklist ~by s k =
+    let rec make state () = match _split ~by s state with
+      | None -> `Nil
+      | Some (state', i, len) ->
+          `Cons (k s i len , make state')
+    in make (SplitAt 0)
+
+  let klist ~by s = _mkklist ~by s _tuple3
+
+  let klist_cpy ~by s = _mkklist ~by s String.sub
+
+  let _mkseq ~by s f k =
+    let rec aux state = match _split ~by s state with
+      | None -> ()
+      | Some (state', i, len) -> k (f s i len); aux state'
+    in aux (SplitAt 0)
+
+  let seq ~by s = _mkseq ~by s _tuple3
+  let seq_cpy ~by s = _mkseq ~by s String.sub
+
+  (*$T
+    Split.list_cpy ~by:"," "aa,bb,cc" = ["aa"; "bb"; "cc"]
+    Split.list_cpy ~by:"--" "a--b----c--" = ["a"; "b"; ""; "c"; ""]
+  *)
+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 repeat s n =
+  assert (n>=0);
+  let len = String.length s in
+  assert(len > 0);
+  let buf = String.create (len * n) in
+  for i = 0 to n-1 do
+    String.blit s 0 buf (i * len) len;
+  done;
+  buf
+
+let prefix ~pre s =
+  String.length pre <= String.length s &&
+  (let i = ref 0 in
+    while !i < String.length pre && s.[!i] = pre.[!i] do incr i done;
+    !i = String.length pre)
+
+let blit = String.blit
+
+let _to_gen s i0 len =
+  let i = ref i0 in
+  fun () ->
+    if !i = i0+len then None
+    else (
+      let c = String.unsafe_get s !i in
+      incr i;
+      Some c
+    )
+
+let to_gen s = _to_gen s 0 (String.length s)
+
+let of_gen g =
+  let b = Buffer.create 32 in
+  let rec aux () = match g () with
+    | None -> Buffer.contents b
+    | Some c -> Buffer.add_char b c; aux ()
+  in aux ()
+
+let to_seq s k = String.iter k s
+
+let of_seq seq =
+  let b= Buffer.create 32 in
+  seq (Buffer.add_char b);
+  Buffer.contents b
+
+let rec _to_klist s i len () =
+  if len=0 then `Nil
+  else `Cons (s.[i], _to_klist s (i+1)(len-1))
+
+let of_klist l =
+  let rec aux acc n l = match l() with
+    | `Nil ->
+        let s = String.create n in
+        let acc = ref acc in
+        for i=n-1 downto 0 do
+          s.[i] <- List.hd !acc;
+          acc := List.tl !acc
+        done;
+        s
+    | `Cons (x,l') -> aux (x::acc) (n+1) l'
+  in aux [] 0 l
+
+let to_klist s = _to_klist s 0 (String.length s)
+
+let pp buf s =
+  Buffer.add_char buf '"';
+  Buffer.add_string buf s;
+  Buffer.add_char buf '"'
+
+module Sub = struct
+  type t = string * int * int
+
+  let make s i ~len =
+    if i<0||len<0||i+len > String.length s then invalid_arg "CCString.Sub.make";
+    s,i,len
+
+  let full s = s, 0, String.length s
+
+  let copy (s,i,len) = String.sub s i len
+
+  let underlying (s,_,_) = s
+
+  let sub (s,i,len) i' len' =
+    if i+i' + len' > i+len then invalid_arg "CCString.Sub.sub";
+    (s, i+i',len')
+
+  let length (_,_,l) = l
+
+  let blit (a1,i1,len1) o1 (a2,i2,len2) o2 len =
+    if o1+len>len1 || o2+len>len2 then invalid_arg "CCString.Sub.blit";
+    String.blit a1 (i1+o1) a2 (i2+o2) len
+
+  let to_gen (s,i,len) = _to_gen s i len
+  let to_seq (s,i,len) k =
+    for i=i to i+len-1 do k s.[i] done
+  let to_klist (s,i,len) = _to_klist s i len
+
+  let pp buf (s,i,len) =
+    Buffer.add_char buf '"';
+    Buffer.add_substring buf s i len;
+    Buffer.add_char buf '"'
+end
diff --git a/core/CCString.mli b/core/CCString.mli
new file mode 100644
index 00000000..65997b77
--- /dev/null
+++ b/core/CCString.mli
@@ -0,0 +1,141 @@
+
+(*
+copyright (c) 2013-2014, simon cruanes
+all rights reserved.
+
+redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+redistributions of source code must retain the above copyright notice, this
+list of conditions and the following disclaimer.  redistributions in binary
+form must reproduce the above copyright notice, this list of conditions and the
+following disclaimer in the documentation and/or other materials provided with
+the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*)
+
+(** {1 Basic String Utils}
+Consider using {!Containers_string.KMP} for pattern search, or Regex
+libraries. *)
+
+type 'a gen = unit -> 'a option
+type 'a sequence = ('a -> unit) -> unit
+type 'a klist = unit -> [`Nil | `Cons of 'a * 'a klist]
+
+(** {2 Common Signature} *)
+
+module type S = sig
+  type t
+
+  val length : t -> int
+
+  val blit : t -> int -> t -> int -> int -> unit
+  (** See {!String.blit} *)
+
+  (** {2 Conversions} *)
+
+  val to_gen : t -> char gen
+
+  val to_seq : t -> char sequence
+
+  val to_klist : t -> char klist
+
+  val pp : Buffer.t -> t -> unit
+end
+
+(** {2 Strings} *)
+
+type t = string
+
+val equal : t -> t -> bool
+
+val compare : t -> t -> int
+
+val hash : t -> int
+
+val of_gen : char gen -> t
+
+val of_seq : char sequence -> t
+
+val of_klist : char klist -> t
+
+val find : ?start:int -> sub:t -> t -> int
+(** Find [sub] in the string, returns its first index or -1.
+    Should only be used with very small [sub] *)
+
+val is_sub : sub:t -> int -> t -> 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],
+    is a substring of [s] starting at position [j] *)
+
+val repeat : t -> int -> t
+(** The same string, repeated n times *)
+
+val prefix : pre:t -> t -> bool
+(** [str_prefix ~pre s] returns [true] iff [pre] is a prefix of [s] *)
+
+include S with type t := t
+
+(** {2 Splitting} *)
+
+module Split : sig
+  val list_ : by:t -> t -> (t*int*int) list
+  (** split the given string along the given separator [by]. Should only
+      be used with very small separators, otherwise
+      use {!Containers_string.KMP}.
+      @return a list of (index,length) of substrings of [s] that are
+      separated by [by]. {!String.sub} can then be used to actually extract
+      the slice.
+      @raise Failure if [by = ""] *)
+
+  val gen : by:t -> t -> (t*int*int) gen
+
+  val seq : by:t -> t -> (t*int*int) sequence
+
+  val klist : by:t -> t -> (t*int*int) klist
+
+  (** {6 Copying functions}
+
+  Those split functions actually copy the substrings, which can be
+  more convenient but less efficient in general *)
+
+  val list_cpy : by:t -> t -> t list
+
+  val gen_cpy : by:t -> t -> t gen
+
+  val seq_cpy : by:t -> t -> t sequence
+
+  val klist_cpy : by:t -> t -> t klist
+end
+
+(** {2 Slices} A contiguous part of a string *)
+
+module Sub : sig
+  type t = string * int * int
+  (** A string, an offset, and the length of the slice *)
+
+  val make : string -> int -> len:int -> t
+
+  val full : string -> t
+  (** Full string *)
+
+  val copy : t -> string
+  (** Make a copy of the substring *)
+
+  val underlying : t -> string
+
+  val sub : t -> int -> int -> t
+  (** Sub-slice *)
+
+  include S with type t := t
+end
diff --git a/string/CCString.ml b/string/CCString.ml
deleted file mode 100644
index a34614cf..00000000
--- a/string/CCString.ml
+++ /dev/null
@@ -1,150 +0,0 @@
-
-(*
-copyright (c) 2013-2014, simon cruanes
-all rights reserved.
-
-redistribution and use in source and binary forms, with or without
-modification, are permitted provided that the following conditions are met:
-
-redistributions of source code must retain the above copyright notice, this
-list of conditions and the following disclaimer.  redistributions in binary
-form must reproduce the above copyright notice, this list of conditions and the
-following disclaimer in the documentation and/or other materials provided with
-the distribution.
-
-THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
-ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
-WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
-DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
-FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
-DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
-SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
-CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
-OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
-OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-*)
-
-(** {1 Basic String Utils} *)
-
-type t = string
-
-let equal a b = a=b
-
-let compare = String.compare
-
-let hash s = Hashtbl.hash s
-
-type 'a gen = unit -> 'a option
-type 'a sequence = ('a -> unit) -> unit
-
-let is_sub ~sub i s j =
-  let rec check k =
-    if i + k = String.length sub
-      then true
-      else sub.[i + k] = s.[j+k] && check (k+1)
-  in
-  check 0
-
-(* note: quite inefficient if [by] is long *)
-let split_gen ~by s =
-  let len_by = String.length by in
-  assert (len_by > 0);
-  let n = String.length s in
-  let prev = ref 0 in
-  let stop = ref false in
-  let rec search i =
-    if !stop then None
-    else if i >= n
-      then (
-        stop := true;
-        Some (String.sub s !prev (n- !prev))  (* done *)
-      )
-    else if is_prefix i 0
-      then (
-        let p = !prev in
-        prev := i+len_by;
-        Some (String.sub s p (i-p))
-      )
-    else search (i+1)
-  and is_prefix i j =
-    if j = len_by
-      then true
-    else if i = n
-      then false
-    else s.[i] = by.[j] && is_prefix (i+1) (j+1)
-  in
-  fun () ->
-    search !prev
-
-let split_seq ~by s k =
-  let rec aux g = match g () with
-    | None -> ()
-    | Some x -> k x; aux g
-  in aux (split_gen ~by s)
-
-let split ~by s =
-  let rec aux g acc = match g () with
-    | None -> List.rev acc
-    | Some x -> aux g (x::acc)
-  in aux (split_gen ~by s) []
-
-(*$T
-  split ~by:"," "aa,bb,cc" = ["aa"; "bb"; "cc"]
-  split ~by:"--" "a--b----c--" = ["a"; "b"; ""; "c"; ""]
-*)
-
-(* 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 then raise Exit;
-      incr i
-    done;
-    -1
-  with Exit ->
-    !i
-
-let repeat s n =
-  assert (n>=0);
-  let len = String.length s in
-  assert(len > 0);
-  let buf = String.create (len * n) in
-  for i = 0 to n-1 do
-    String.blit s 0 buf (i * len) len;
-  done;
-  buf
-
-let prefix ~pre s =
-  String.length pre <= String.length s &&
-  (let i = ref 0 in
-    while !i < String.length pre && s.[!i] = pre.[!i] do incr i done;
-    !i = String.length pre)
-
-
-let to_gen s =
-  let i = ref 0 in
-  fun () ->
-    if !i = String.length s then None
-    else (
-      let c = String.unsafe_get s !i in
-      incr i;
-      Some c
-    )
-
-let of_gen g =
-  let b = Buffer.create 32 in
-  let rec aux () = match g () with
-    | None -> Buffer.contents b
-    | Some c -> Buffer.add_char b c; aux ()
-  in aux ()
-
-let to_seq s k = String.iter k s
-
-let of_seq seq =
-  let b= Buffer.create 32 in
-  seq (Buffer.add_char b);
-  Buffer.contents b
-
-let pp = Buffer.add_string
diff --git a/string/CCString.mli b/string/CCString.mli
deleted file mode 100644
index 50ba1bcf..00000000
--- a/string/CCString.mli
+++ /dev/null
@@ -1,70 +0,0 @@
-
-(*
-copyright (c) 2013-2014, simon cruanes
-all rights reserved.
-
-redistribution and use in source and binary forms, with or without
-modification, are permitted provided that the following conditions are met:
-
-redistributions of source code must retain the above copyright notice, this
-list of conditions and the following disclaimer.  redistributions in binary
-form must reproduce the above copyright notice, this list of conditions and the
-following disclaimer in the documentation and/or other materials provided with
-the distribution.
-
-THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
-ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
-WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
-DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
-FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
-DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
-SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
-CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
-OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
-OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-*)
-
-(** {1 Basic String Utils}
-Consider using KMP instead. *)
-
-type t = string
-
-val equal : t -> t -> bool
-
-val compare : t -> t -> int
-
-val hash : t -> int
-
-type 'a gen = unit -> 'a option
-type 'a sequence = ('a -> unit) -> unit
-
-val is_sub : sub:t -> int -> t -> int -> bool
-(** [is_sub ~sub i s j] returns [true] iff [sub] is a substring of [s] starting
-    at position [j] *)
-
-val split : by:t -> t -> t list
-(** split the given string along the given separator [by]. Should only
-    be used with very small separators, otherwise use {!KMP}.
-    @raise Failure if [by = ""] *)
-
-val split_gen : by:t -> t -> t gen
-
-val split_seq : by:t -> t -> t sequence
-
-val find : ?start:int -> sub:t -> t -> int
-(** Find [sub] in the string, returns its first index or -1.
-    Should only be used with very small [sub] *)
-
-val repeat : t -> int -> t
-(** The same string, repeated n times *)
-
-val prefix : pre:t -> t -> bool
-(** [str_prefix ~pre s] returns [true] iff [pre] is a prefix of [s] *)
-
-val to_gen : t -> char gen
-val of_gen : char gen -> t
-
-val to_seq : t -> char sequence
-val of_seq : char sequence -> t
-
-val pp : Buffer.t -> t -> unit