diff --git a/src/core/CCString.cppo.ml b/src/core/CCString.cppo.ml
index 7fd6e70e..4a176eb9 100644
--- a/src/core/CCString.cppo.ml
+++ b/src/core/CCString.cppo.ml
@@ -116,6 +116,39 @@ let rfind ~sub s =
   with Exit ->
     !i
 
+(* replace substring [s.[pos]....s.[pos+len-1]] by [by] in [s] *)
+let replace_at_ ~pos ~len ~by s =
+  let b = Buffer.create (length s + length by - len) in
+  Buffer.add_substring b s 0 pos;
+  Buffer.add_string b by;
+  Buffer.add_substring b s (pos+len) (String.length s - pos - len);
+  Buffer.contents b
+
+let replace ?(which=`All) ~sub ~by s = match which with
+  | `Left ->
+      let i = find ~sub s in
+      if i>=0 then replace_at_ ~pos:i ~len:(String.length sub) ~by s else s
+  | `Right ->
+      let i = rfind ~sub s in
+      if i>=0 then replace_at_ ~pos:i ~len:(String.length sub) ~by s else s
+  | `All ->
+      let b = Buffer.create (String.length s) in
+      let start = ref 0 in
+      while !start < String.length s do
+        let i = find ~start:!start ~sub s in
+        if i>=0 then (
+          (* between last and cur occurrences *)
+          Buffer.add_substring b s !start (i- !start);
+          Buffer.add_string b by;
+          start := i + String.length sub
+        ) else (
+          (* add remainder *)
+          Buffer.add_substring b s !start (String.length s - !start);
+          start := String.length s (* stop *)
+        )
+      done;
+      Buffer.contents b
+
 module Split = struct
   type split_state =
     | SplitStop
diff --git a/src/core/CCString.mli b/src/core/CCString.mli
index e6b86ff1..4e00f8a0 100644
--- a/src/core/CCString.mli
+++ b/src/core/CCString.mli
@@ -129,6 +129,24 @@ val rfind : sub:string -> string -> int
   rfind ~sub:"bc" "abcdbcd" = 4
 *)
 
+val replace : ?which:[`Left|`Right|`All] -> sub:string -> by:string -> string -> string
+(** [replace ~sub ~by s] replaces some occurrences of [sub] by [by] in [s]
+    @param which decides whether the occurrences to replace are:
+      {ul
+        {il [`Left] first occurrence from the left (beginning)}
+        {il [`Right] first occurrence from the right (end)}
+        {il [`All] all occurrences (default)}
+      }
+    @since NEXT_RELEASE *)
+
+(*$= & ~printer:CCFun.id
+  (replace ~which:`All ~sub:"a" ~by:"b" "abcdabcd") "bbcdbbcd"
+  (replace ~which:`Left ~sub:"a" ~by:"b" "abcdabcd") "bbcdabcd"
+  (replace ~which:`Right ~sub:"a" ~by:"b" "abcdabcd") "abcdbbcd"
+  (replace ~which:`All ~sub:"ab" ~by:"hello" "  abab cdabb a") \
+    "  hellohello cdhellob a"
+*)
+
 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] *)