diff --git a/src/core/CCString.cppo.ml b/src/core/CCString.cppo.ml
index 6097665a..dbd84898 100644
--- a/src/core/CCString.cppo.ml
+++ b/src/core/CCString.cppo.ml
@@ -367,14 +367,18 @@ module Split = struct
   let left_exn ~by s =
     let i = find ~sub:by s in
     if i = ~-1 then raise Not_found
-    else String.sub s 0 i, String.sub s (i+1) (String.length s - i - 1)
+    else
+      let right = i + String.length by in
+      String.sub s 0 i, String.sub s right (String.length s - right)
 
   let left ~by s = try Some (left_exn ~by s) with Not_found -> None
 
   let right_exn ~by s =
     let i = rfind ~sub:by s in
     if i = ~-1 then raise Not_found
-    else String.sub s 0 i, String.sub s (i+1) (String.length s - i - 1)
+    else
+      let right = i + String.length by in
+      String.sub s 0 i, String.sub s right (String.length s - right)
 
   let right ~by s = try Some (right_exn ~by s) with Not_found -> None
 end
diff --git a/src/core/CCString.mli b/src/core/CCString.mli
index d0a2c5c2..edb10378 100644
--- a/src/core/CCString.mli
+++ b/src/core/CCString.mli
@@ -449,7 +449,9 @@ module Split : sig
 
   (*$T
     Split.left ~by:" " "ab cde f g " = Some ("ab", "cde f g ")
+    Split.left ~by:"__" "a__c__e_f" = Some ("a", "c__e_f")
     Split.left ~by:"_" "abcde" = None
+    Split.left ~by:"a_" "abcde" = None
   *)
 
   val right : by:string -> string -> (string * string) option
@@ -464,7 +466,9 @@ module Split : sig
 
   (*$T
     Split.right ~by:" " "ab cde f g" = Some ("ab cde f", "g")
+    Split.right ~by:"__" "a__c__e_f" = Some ("a__c", "e_f")
     Split.right ~by:"_" "abcde" = None
+    Split.right ~by:"a_" "abcde" = None
   *)
 end