diff --git a/doc/content/xapi/cli/_index.md b/doc/content/xapi/cli/_index.md
index 6715e7288c0..093e1576aaf 100644
--- a/doc/content/xapi/cli/_index.md
+++ b/doc/content/xapi/cli/_index.md
@@ -156,7 +156,7 @@ So each function receives a printer for sending text output to the xe client, an
       let mac = List.assoc_default "mac" params "" in
       let network = Client.Network.get_by_uuid rpc session_id network in
       let pifs = List.assoc "pif-uuids" params in
-      let uuids = String.split ',' pifs in
+      let uuids = String.split_on_char ',' pifs in
       let pifs = List.map (fun uuid -> Client.PIF.get_by_uuid rpc session_id uuid) uuids in
       let mode = Record_util.bond_mode_of_string (List.assoc_default "mode" params "") in
       let properties = read_map_params "properties" params in
diff --git a/ocaml/database/parse_db_conf.ml b/ocaml/database/parse_db_conf.ml
index 67aa5c70d80..d1c7482c080 100644
--- a/ocaml/database/parse_db_conf.ml
+++ b/ocaml/database/parse_db_conf.ml
@@ -13,7 +13,6 @@
  *)
 (* !!! This needs to be moved out of xapi and into the database directory; probably being merged with db_connections !!! *)
 
-open Xapi_stdext_std.Xstringext
 open Xapi_stdext_unix
 
 module D = Debug.Make (struct let name = "parse_db_conf" end)
@@ -110,7 +109,7 @@ let parse_db_conf s =
     let conf = Unixext.string_of_file s in
     let lines : string list ref = ref [] in
     let consume_line () = lines := List.tl !lines in
-    lines := String.split '\n' conf ;
+    lines := String.split_on_char '\n' conf ;
     List.iter (fun line -> debug "%s" line) !lines ;
     let read_block () =
       let path_line = List.hd !lines in
@@ -120,7 +119,7 @@ let parse_db_conf s =
       while !lines <> [] && List.hd !lines <> "" do
         let line = List.hd !lines in
         key_values :=
-          ( match String.split ':' line with
+          ( match String.split_on_char ':' line with
           | k :: vs ->
               ( String.lowercase_ascii k
               , String.lowercase_ascii (String.concat ":" vs)
diff --git a/ocaml/database/redo_log.ml b/ocaml/database/redo_log.ml
index 8c2c95928d7..42175580f83 100644
--- a/ocaml/database/redo_log.ml
+++ b/ocaml/database/redo_log.ml
@@ -12,7 +12,6 @@
  * GNU Lesser General Public License for more details.
  *)
 open Xapi_stdext_pervasives.Pervasiveext
-open Xapi_stdext_std.Xstringext
 open Xapi_stdext_unix
 
 let with_lock = Xapi_stdext_threads.Threadext.Mutex.execute
diff --git a/ocaml/doc/dune b/ocaml/doc/dune
index 061ba778232..618de9e30fb 100644
--- a/ocaml/doc/dune
+++ b/ocaml/doc/dune
@@ -1,7 +1,7 @@
 (executable
   (modes exe)
   (name jsapi)
-  (libraries    
+  (libraries
     mustache
     rpclib.core
     rpclib.json
@@ -10,7 +10,6 @@
     xapi-consts
     xapi-datamodel
     xapi-stdext-pervasives
-    xapi-stdext-std
     xapi-stdext-unix
   )
   (preprocess (pps ppx_deriving_rpc))
diff --git a/ocaml/doc/jsapi.ml b/ocaml/doc/jsapi.ml
index fa891e57743..1fef7fabf00 100644
--- a/ocaml/doc/jsapi.ml
+++ b/ocaml/doc/jsapi.ml
@@ -12,7 +12,6 @@
  * GNU Lesser General Public License for more details.
  *)
 
-open Xapi_stdext_std.Xstringext
 open Xapi_stdext_pervasives.Pervasiveext
 module Unixext = Xapi_stdext_unix.Unixext
 open Datamodel_types
diff --git a/ocaml/idl/markdown_backend.ml b/ocaml/idl/markdown_backend.ml
index ca067ef9dcb..3125d850b85 100644
--- a/ocaml/idl/markdown_backend.ml
+++ b/ocaml/idl/markdown_backend.ml
@@ -43,41 +43,41 @@ let compare_case_ins x y =
   compare (String.lowercase_ascii x) (String.lowercase_ascii y)
 
 let escape s =
-  let esc_char = function
+  let replace = function
     | '\\' ->
-        "&#92;"
+        Some "&#92;"
     | '*' ->
-        "&#42;"
+        Some "&#42;"
     | '_' ->
-        "&#95;"
+        Some "&#95;"
     | '{' ->
-        "&#123;"
+        Some "&#123;"
     | '}' ->
-        "&#125;"
+        Some "&#125;"
     | '[' ->
-        "&#91;"
+        Some "&#91;"
     | ']' ->
-        "&#93;"
+        Some "&#93;"
     | '(' ->
-        "&#40;"
+        Some "&#40;"
     | ')' ->
-        "&#41;"
+        Some "&#41;"
     | '>' ->
-        "&gt;"
+        Some "&gt;"
     | '<' ->
-        "&lt;"
+        Some "&lt;"
     | '#' ->
-        "&#35;"
+        Some "&#35;"
     | '+' ->
-        "&#43;"
+        Some "&#43;"
     | '-' ->
-        "&#45;"
+        Some "&#45;"
     | '!' ->
-        "&#33;"
-    | c ->
-        String.make 1 c
+        Some "&#33;"
+    | _ ->
+        None
   in
-  String.to_seq s |> Seq.map esc_char |> List.of_seq |> String.concat ""
+  Xapi_stdext_std.Xstringext.String.replaced ~replace s
 
 let rec of_ty_verbatim = function
   | SecretString | String ->
diff --git a/ocaml/idl/ocaml_backend/gen_rbac.ml b/ocaml/idl/ocaml_backend/gen_rbac.ml
index 7914dba96dd..ad17e10db5f 100644
--- a/ocaml/idl/ocaml_backend/gen_rbac.ml
+++ b/ocaml/idl/ocaml_backend/gen_rbac.ml
@@ -70,11 +70,10 @@ let role_uuid name = Option.get (hash2uuid name)
 let permission_description = "A basic permission"
 
 let permission_name wire_name =
-  let open Xapi_stdext_std in
   let s1 = replace_char (Printf.sprintf "permission_%s" wire_name) '.' '_' in
   let s2 = replace_char s1 '/' '_' in
-  let s3 = Xstringext.String.replace "*" "WILDCHAR" s2 in
-  Xstringext.String.replace ":" "_" s3
+  let s3 = Xapi_stdext_std.Xstringext.String.replace '*' ~by:"WILDCHAR" s2 in
+  replace_char s3 ':' '_'
 
 let permission_index = ref 0
 
diff --git a/ocaml/libs/http-lib/dune b/ocaml/libs/http-lib/dune
index 4e8d255b6bd..ac6db1b23d4 100644
--- a/ocaml/libs/http-lib/dune
+++ b/ocaml/libs/http-lib/dune
@@ -63,6 +63,7 @@
   xapi-backtrace
   xapi-log
   xapi-stdext-pervasives
+  xapi-stdext-std
   xapi-stdext-threads
   xapi-stdext-unix))
 
diff --git a/ocaml/libs/http-lib/http_svr.ml b/ocaml/libs/http-lib/http_svr.ml
index 7c5ecc393d1..6271c5ad987 100644
--- a/ocaml/libs/http-lib/http_svr.ml
+++ b/ocaml/libs/http-lib/http_svr.ml
@@ -331,28 +331,22 @@ module Server = struct
       x.handlers []
 end
 
-let escape uri =
-  (* from xapi-stdext-std xstringext *)
-  let escaped ~rules string =
-    let aux h t =
-      ( if List.mem_assoc h rules then
-          List.assoc h rules
-        else
-          Astring.String.of_char h
+let escape_html uri =
+  Xapi_stdext_std.Xstringext.String.replaced
+    ~replace:(function
+      | '<' ->
+          Some "&lt;"
+      | '>' ->
+          Some "&gt;"
+      | '\'' ->
+          Some "&apos;"
+      | '"' ->
+          Some "&quot;"
+      | '&' ->
+          Some "&amp;"
+      | _ ->
+          None
       )
-      :: t
-    in
-    String.concat "" (Astring.String.fold_right aux string [])
-  in
-  escaped
-    ~rules:
-      [
-        ('<', "&lt;")
-      ; ('>', "&gt;")
-      ; ('\'', "&apos;")
-      ; ('"', "&quot;")
-      ; ('&', "&amp;")
-      ]
     uri
 
 exception Generic_error of string
@@ -508,7 +502,7 @@ let read_request ?proxy_seen ~read_timeout ~total_timeout ~max_length fd =
                 )
         | exc ->
             response_internal_error exc fd
-              ~extra:(escape (Printexc.to_string exc))
+              ~extra:(escape_html (Printexc.to_string exc))
     ) ;
     (None, None)
 
@@ -557,7 +551,7 @@ let handle_one (x : 'a Server.t) ss context req =
                 )
         | exc ->
             response_internal_error ~req exc ss
-              ~extra:(escape (Printexc.to_string exc))
+              ~extra:(escape_html (Printexc.to_string exc))
     ) ;
     !finished
 
diff --git a/ocaml/libs/http-lib/http_svr.mli b/ocaml/libs/http-lib/http_svr.mli
index 61b49e7b3f2..cf335592d7b 100644
--- a/ocaml/libs/http-lib/http_svr.mli
+++ b/ocaml/libs/http-lib/http_svr.mli
@@ -68,6 +68,9 @@ val start :
 
 val handle_one : 'a Server.t -> Unix.file_descr -> 'a -> Http.Request.t -> bool
 
+val escape_html : string -> string
+(** Escapes HTML: replaces characters with their character references *)
+
 exception Socket_not_found
 
 val stop : socket -> unit
diff --git a/ocaml/libs/sexpr/sExpr.ml b/ocaml/libs/sexpr/sExpr.ml
index 3637ac6abf5..c9794265d49 100644
--- a/ocaml/libs/sexpr/sExpr.ml
+++ b/ocaml/libs/sexpr/sExpr.ml
@@ -28,7 +28,7 @@ let is_escape_char = function '\\' | '\'' -> true | _ -> false
 (* XXX: This escapes "'c'" and "\'c\'" to "\\'c\\'".
  * They are both unescaped as "'c'". They have been ported
  * to make sure that this corner case is left unchanged.
- * It is worth investigating the use of 
+ * It is worth investigating the use of
  * - Astring.String.Ascii.escape_string
  * - Astring.String.Ascii.unescape
  * that have guaranteed invariants and optimised performances *)
diff --git a/ocaml/libs/tgroup/dune b/ocaml/libs/tgroup/dune
index 6c152a5c2ef..12070176651 100644
--- a/ocaml/libs/tgroup/dune
+++ b/ocaml/libs/tgroup/dune
@@ -2,7 +2,7 @@
  (name tgroup)
  (modules tgroup)
  (public_name tgroup)
- (libraries unix xapi-log xapi-stdext-unix xapi-stdext-std))
+ (libraries astring unix xapi-log xapi-stdext-unix))
 
 (test
  (name test_tgroup)
diff --git a/ocaml/libs/tgroup/tgroup.ml b/ocaml/libs/tgroup/tgroup.ml
index 071a9dfe0d2..e5b163fea60 100644
--- a/ocaml/libs/tgroup/tgroup.ml
+++ b/ocaml/libs/tgroup/tgroup.ml
@@ -70,8 +70,7 @@ module Group = struct
       | _ ->
           false
 
-    let sanitize s =
-      Xapi_stdext_std.Xstringext.String.filter_chars s is_alphanum
+    let sanitize s = Astring.String.filter is_alphanum s
 
     let make ?user_agent subject_sid =
       let user_agent =
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/bench/bench_xstringext.ml b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/bench/bench_xstringext.ml
index 52897ee2a0c..d302737b1b1 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/bench/bench_xstringext.ml
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/bench/bench_xstringext.ml
@@ -8,36 +8,42 @@ let make_string len = String.init len (fun i -> Char.chr (33 + (i mod 94)))
 let escape_rules =
   [('a', "[A]"); ('e', "[E]"); ('i', "[I]"); ('o', "[O]"); ('u', "[U]")]
 
-(* Reference implementation from xstringext_test.ml *)
-let escaped_spec ?rules string =
-  match rules with
-  | None ->
-      String.escaped string
-  | Some rules ->
-      let apply_rules char =
-        match List.assoc_opt char rules with
-        | None ->
-            Seq.return char
-        | Some replacement ->
-            String.to_seq replacement
-      in
-      string |> String.to_seq |> Seq.concat_map apply_rules |> String.of_seq
+let replace = function
+  | 'a' ->
+      Some "[A]"
+  | 'e' ->
+      Some "[E]"
+  | 'i' ->
+      Some "[I]"
+  | 'o' ->
+      Some "[O]"
+  | 'u' ->
+      Some "[U]"
+  | _ ->
+      None
+
+(* Reference implementation using lists *)
+let replaced_spec ~rules string =
+  let apply_rules char = List.assoc_opt char rules in
+  XString.replaced ~replace:apply_rules string
 
-let escaped_benchmark n =
+let replaced ~rules string = XString.replaced ~replace:rules string
+
+let replaced_benchmark n =
   let s = make_string n in
-  Staged.stage @@ fun () -> ignore (XString.escaped ~rules:escape_rules s)
+  Staged.stage @@ fun () -> ignore (replaced ~rules:replace s)
 
-let escaped_spec_benchmark n =
+let replaced_spec_benchmark n =
   let s = make_string n in
-  Staged.stage @@ fun () -> ignore (escaped_spec ~rules:escape_rules s)
+  Staged.stage @@ fun () -> ignore (replaced_spec ~rules:escape_rules s)
 
-let test_escaped =
-  Test.make_indexed ~name:"escaped" ~fmt:"%s %d" ~args:[100; 500; 1000]
-    escaped_benchmark
+let test_replaced =
+  Test.make_indexed ~name:"replaced" ~fmt:"%s %d" ~args:[100; 500; 1000]
+    replaced_benchmark
 
-let test_escaped_spec =
-  Test.make_indexed ~name:"escaped-spec" ~fmt:"%s %d" ~args:[100; 500; 1000]
-    escaped_spec_benchmark
+let test_replaced_spec =
+  Test.make_indexed ~name:"replaced-spec" ~fmt:"%s %d" ~args:[100; 500; 1000]
+    replaced_spec_benchmark
 
 let benchmark () =
   let ols =
@@ -50,8 +56,8 @@ let benchmark () =
     Benchmark.cfg ~limit:2000 ~quota:(Time.second 0.5) ~kde:(Some 1000) ()
   in
   let test =
-    Test.make_grouped ~name:"escaped-comparison"
-      [test_escaped; test_escaped_spec]
+    Test.make_grouped ~name:"replaced-comparison"
+      [test_replaced; test_replaced_spec]
   in
   let raw_results = Benchmark.all cfg instances test in
   let results =
@@ -97,8 +103,10 @@ let () =
   List.iter
     (fun size ->
       Printf.printf "String size %s:\n" size ;
-      let opt_test = Printf.sprintf "escaped-comparison/escaped %s" size in
-      let ref_test = Printf.sprintf "escaped-comparison/escaped-spec %s" size in
+      let opt_test = Printf.sprintf "replaced-comparison/replaced %s" size in
+      let ref_test =
+        Printf.sprintf "replaced-comparison/replaced-spec %s" size
+      in
       match (get_timing opt_test, get_timing ref_test) with
       | Some opt_time, Some ref_time ->
           let improvement = (ref_time -. opt_time) /. ref_time *. 100.0 in
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext.ml b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext.ml
index 4e5379d7b36..854245b9b6d 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext.ml
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext.ml
@@ -12,79 +12,18 @@
  * GNU Lesser General Public License for more details.
  *)
 module String = struct
-  include String
-
-  let of_char c = String.make 1 c
-
-  let rev_map f string =
-    let n = length string in
-    String.init n (fun i -> f string.[n - i - 1])
-
-  let rev_iter f string =
-    for i = length string - 1 downto 0 do
-      f string.[i]
-    done
-
-  let fold_left f accu string =
-    let accu = ref accu in
-    for i = 0 to length string - 1 do
-      accu := f !accu string.[i]
-    done ;
-    !accu
-
-  let fold_right f string accu =
-    let accu = ref accu in
-    for i = length string - 1 downto 0 do
-      accu := f string.[i] !accu
-    done ;
-    !accu
-
-  (** Returns true for whitespace characters, false otherwise *)
-  let isspace = function ' ' | '\n' | '\r' | '\t' -> true | _ -> false
-
-  let split_f p str =
-    let split_one seq =
-      let not_p c = not (p c) in
-      let a = Seq.take_while not_p seq in
-      let b = Seq.drop_while not_p seq in
-      (a, b)
-    in
-    let drop seq = Seq.drop_while p seq in
-    let rec split acc chars =
-      if Seq.is_empty chars then
-        acc
-      else
-        let a, b = split_one chars in
-        let b = drop b in
-        let acc = if Seq.is_empty a then acc else Seq.cons a acc in
-        split acc b
-    in
-    String.to_seq str
-    |> split Seq.empty
-    |> Seq.map String.of_seq
-    |> List.of_seq
-    |> List.rev
-
-  let index_opt s c =
-    let rec loop i =
-      if String.length s = i then
-        None
-      else if s.[i] = c then
-        Some i
-      else
-        loop (i + 1)
-    in
-    loop 0
+  let sub_to_end s start =
+    let length = String.length s in
+    String.sub s start (length - start)
 
-  let rec split ?(limit = -1) c s =
-    let i = match index_opt s c with Some x -> x | None -> -1 in
-    let nlimit = if limit = -1 || limit = 0 then limit else limit - 1 in
-    if i = -1 || nlimit = 0 then
-      [s]
-    else
-      let a = String.sub s 0 i
-      and b = String.sub s (i + 1) (String.length s - i - 1) in
-      a :: split ~limit:nlimit c b
+  let rec split ~limit sep s =
+    match (String.index_opt s sep, limit < 2) with
+    | None, _ | _, true ->
+        [s]
+    | Some pos, false ->
+        let first = String.sub s 0 pos in
+        let rest = sub_to_end s (pos + 1) in
+        first :: split ~limit:(limit - 1) sep rest
 
   let rtrim s =
     let n = String.length s in
@@ -93,71 +32,6 @@ module String = struct
     else
       s
 
-  (** has_substr str sub returns true if sub is a substring of str. Simple, naive, slow. *)
-  let has_substr str sub =
-    if String.length sub > String.length str then
-      false
-    else
-      let result = ref false in
-      for start = 0 to String.length str - String.length sub do
-        if String.sub str start (String.length sub) = sub then result := true
-      done ;
-      !result
-
-  (** find all occurences of needle in haystack and return all their respective index *)
-  let find_all needle haystack =
-    let m = String.length needle and n = String.length haystack in
-    if m > n then
-      []
-    else
-      let i = ref 0 and found = ref [] in
-      while !i < n - m + 1 do
-        if String.sub haystack !i m = needle then (
-          found := !i :: !found ;
-          i := !i + m
-        ) else
-          incr i
-      done ;
-      List.rev !found
-
-  (* replace all @f substring in @s by @t *)
-  let replace f t s =
-    let indexes = find_all f s in
-    let n = List.length indexes in
-    if n > 0 then (
-      let len_f = String.length f and len_t = String.length t in
-      let new_len = String.length s + (n * len_t) - (n * len_f) in
-      let new_b = Bytes.make new_len '\000' in
-      let orig_offset = ref 0 and dest_offset = ref 0 in
-      List.iter
-        (fun h ->
-          let len = h - !orig_offset in
-          Bytes.blit_string s !orig_offset new_b !dest_offset len ;
-          Bytes.blit_string t 0 new_b (!dest_offset + len) len_t ;
-          orig_offset := !orig_offset + len + len_f ;
-          dest_offset := !dest_offset + len + len_t
-        )
-        indexes ;
-      Bytes.blit_string s !orig_offset new_b !dest_offset
-        (String.length s - !orig_offset) ;
-      Bytes.unsafe_to_string new_b
-    ) else
-      s
-
-  let filter_chars s valid =
-    let badchars = ref false in
-    let buf = Buffer.create 0 in
-    for i = 0 to String.length s - 1 do
-      if !badchars then (
-        if valid s.[i] then
-          Buffer.add_char buf s.[i]
-      ) else if not (valid s.[i]) then (
-        Buffer.add_substring buf s 0 i ;
-        badchars := true
-      )
-    done ;
-    if !badchars then Buffer.contents buf else s
-
   let map_unlikely s f =
     let changed = ref false in
     let m = ref 0 in
@@ -178,18 +52,10 @@ module String = struct
     ) else
       s
 
-  let escaped ?rules s =
-    match rules with
-    | None ->
-        String.escaped s
-    | Some rules ->
-        map_unlikely s (fun c -> List.assoc_opt c rules)
-
-  let sub_to_end s start =
-    let length = String.length s in
-    String.sub s start (length - start)
-
-  let sub_before c s = String.sub s 0 (String.index s c)
+  let replace char ~by s =
+    let replaceable = Stdlib.Char.equal char in
+    let get_replacement c = if replaceable c then Some by else None in
+    map_unlikely s get_replacement
 
-  let sub_after c s = sub_to_end s (String.index s c + 1)
+  let replaced ~replace s = map_unlikely s replace
 end
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext.mli b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext.mli
index 1f27490493d..62a1053d883 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext.mli
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext.mli
@@ -12,63 +12,21 @@
  * GNU Lesser General Public License for more details.
  *)
 module String : sig
-  include module type of String
+  val replaced : replace:(char -> string option) -> string -> string
+  (** [replaced ~replacement str] applies [replace] to all characters in [str]
+      and when it returns [Some rep] the character is replaced with [rep] in
+      the resulting string *)
 
-  val of_char : char -> string
+  val replace : char -> by:string -> string -> string
+  (** [replace ch ~by s] replaces all the occurrences of [ch] in [s] by [~by]
+    *)
 
-  val rev_map : (char -> char) -> string -> string
-  (** Map a string to a string, applying the given function in reverse
-      order. *)
-
-  val rev_iter : (char -> unit) -> string -> unit
-  (** Iterate over the characters in a string in reverse order. *)
-
-  val fold_left : ('a -> char -> 'a) -> 'a -> string -> 'a
-  (** Fold over the characters in a string. *)
-
-  val fold_right : (char -> 'a -> 'a) -> string -> 'a -> 'a
-  (** Iterate over the characters in a string in reverse order. *)
-
-  val isspace : char -> bool
-  (** True if the character is whitespace *)
-
-  val escaped : ?rules:(char * string) list -> string -> string
-  (** Backward-compatible string escaping, defaulting to the built-in
-      	OCaml string escaping but allowing an arbitrary mapping from characters
-      	to strings. *)
-
-  val split_f : (char -> bool) -> string -> string list
-  (** Take a predicate and a string, return a list of strings separated by
-      runs of characters where the predicate was true. Avoid if possible, it's
-      very costly to execute. *)
-
-  val split : ?limit:int -> char -> string -> string list
+  val split : limit:int -> char -> string -> string list
   (** split a string on a single char *)
 
   val rtrim : string -> string
   (** FIXME document me|remove me if similar to strip *)
 
-  val has_substr : string -> string -> bool
-  (** True if sub is a substr of str *)
-
-  val find_all : string -> string -> int list
-  (** find all occurences of needle in haystack and return all their respective index *)
-
-  val replace : string -> string -> string -> string
-  (** replace all [f] substring in [s] by [t] *)
-
-  val filter_chars : string -> (char -> bool) -> string
-  (** filter chars from a string *)
-
-  val map_unlikely : string -> (char -> string option) -> string
-  (** map a string trying to fill the buffer by chunk *)
-
   val sub_to_end : string -> int -> string
   (** a substring from the specified position to the end of the string *)
-
-  val sub_before : char -> string -> string
-  (** a substring from the start of the string to the first occurrence of a given character, excluding the character *)
-
-  val sub_after : char -> string -> string
-  (** a substring from  the first occurrence of a given character to the end of the string, excluding the character *)
 end
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext_test.ml b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext_test.ml
index 9b7eb2674a1..4fe39225785 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext_test.ml
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-std/xstringext_test.ml
@@ -13,10 +13,6 @@
 
 module XString = Xapi_stdext_std.Xstringext.String
 
-let test_boolean tested_f (name, case, expected) =
-  let check () = Alcotest.(check bool) name expected (tested_f case) in
-  (name, `Quick, check)
-
 let test_string tested_f (name, case, expected) =
   let check () = Alcotest.(check string) name expected (tested_f case) in
   (name, `Quick, check)
@@ -27,47 +23,14 @@ let test_list tested_f (name, case, expected) =
   in
   (name, `Quick, check)
 
-let test_rev_map =
-  let spec_rev = [("", ""); ("foo bar", "rab oof")] in
-  let spec_func = [("id", Fun.id); ("uppercase_ascii", Char.uppercase_ascii)] in
-  let test (f_name, f) (case, expected) =
-    let expected = String.map f expected in
-    let name =
-      Printf.sprintf {|"%s" produces "%s" (%s)|} case expected f_name
-    in
-    test_string (XString.rev_map f) (name, case, expected)
-  in
-  let tests =
-    (* Generate the product of the two lists to generate the tests *)
-    List.concat_map (fun func -> List.map (test func) spec_rev) spec_func
-  in
-  ("rev_map", tests)
-
 let test_split =
-  let test ?limit (splitter, splitted, expected) =
-    let split, name =
-      match limit with
-      | None ->
-          let name = Printf.sprintf {|'%c' splits "%s"|} splitter splitted in
-          (* limit being set to -1 is the same as not using the parameter *)
-          let split = XString.split ~limit:(-1) in
-          (split, name)
-      | Some limit ->
-          let name =
-            Printf.sprintf {|'%c' splits "%s" with limit %i|} splitter splitted
-              limit
-          in
-          let split = XString.split ~limit in
-          (split, name)
+  let test limit (splitter, splitted, expected) =
+    let split = XString.split ~limit in
+    let name =
+      Printf.sprintf {|'%c' splits "%s" with limit %i|} splitter splitted limit
     in
     test_list (split splitter) (name, splitted, expected)
   in
-  let specs_no_limit =
-    [
-      ('.', "...", [""; ""; ""; ""]); ('.', "foo.bar.baz", ["foo"; "bar"; "baz"])
-    ]
-  in
-  let tests_no_limit = List.map test specs_no_limit in
   let specs_limit =
     [
       (0, [('.', "...", ["..."]); ('.', "foo.bar.baz", ["foo.bar.baz"])])
@@ -82,50 +45,12 @@ let test_split =
     ; (4, [('.', "...", [""; ""; ""; ""])])
     ]
   in
-  let tests_limit =
+  let tests =
     List.concat_map
-      (fun (limit, spec) -> List.map (test ~limit) spec)
+      (fun (limit, spec) -> List.map (test limit) spec)
       specs_limit
   in
-  ("split", List.concat [tests_no_limit; tests_limit])
-
-let test_split_f =
-  let specs =
-    [
-      (XString.isspace, "foo bar", ["foo"; "bar"])
-    ; (XString.isspace, "foo  bar", ["foo"; "bar"])
-    ; (XString.isspace, "foo \n\t\r bar", ["foo"; "bar"])
-    ; (XString.isspace, " foo bar ", ["foo"; "bar"])
-    ; (XString.isspace, "", [])
-    ; (XString.isspace, " ", [])
-    ]
-  in
-  let test (splitter, splitted, expected) =
-    let name = Printf.sprintf {|"%s"|} (String.escaped splitted) in
-    test_list (XString.split_f splitter) (name, splitted, expected)
-  in
-  let tests = List.map test specs in
-  ("split_f", tests)
-
-let test_has_substr =
-  let spec =
-    [
-      ("", "", true)
-    ; ("", "foo bar", true)
-    ; ("f", "foof", true)
-    ; ("foofo", "foof", false)
-    ; ("foof", "foof", true)
-    ; ("f", "foof", true)
-    ; ("fo", "foof", true)
-    ; ("of", "foof", true)
-    ; ("ff", "foof", false)
-    ]
-  in
-  let test (contained, container, expected) =
-    let name = Printf.sprintf {|"%s" in "%s"|} contained container in
-    test_boolean (XString.has_substr container) (name, contained, expected)
-  in
-  ("has_substr", List.map test spec)
+  ("split", tests)
 
 let test_rtrim =
   let spec =
@@ -147,49 +72,4 @@ let test_rtrim =
   in
   ("rtrim", List.map test spec)
 
-(** Simple implementation of escaped for testing against *)
-let escaped_spec ?rules string =
-  match rules with
-  | None ->
-      String.escaped string
-  | Some rules ->
-      let apply_rules char =
-        match List.assoc_opt char rules with
-        | None ->
-            Seq.return char
-        | Some replacement ->
-            String.to_seq replacement
-      in
-      string |> String.to_seq |> Seq.concat_map apply_rules |> String.of_seq
-
-let test_escaped =
-  let open QCheck2 in
-  (* Generator for escape rules: list of (char, string) mappings *)
-  let gen_rules =
-    let open Gen in
-    let gen_rule = pair char (string_size (int_range 0 5) ~gen:char) in
-    list gen_rule
-  in
-  (* Generator for test input: string and optional rules *)
-  let gen_input = Gen.pair Gen.string (Gen.opt gen_rules) in
-  let property (s, rules) =
-    let expected = escaped_spec ?rules s in
-    let actual = XString.escaped ?rules s in
-    String.equal expected actual
-  in
-  let test =
-    Test.make ~name:"escaped matches reference implementation" ~count:1000
-      gen_input property
-  in
-  ("escaped", [QCheck_alcotest.to_alcotest test])
-
-let () =
-  Alcotest.run "Xstringext"
-    [
-      test_rev_map
-    ; test_split
-    ; test_split_f
-    ; test_has_substr
-    ; test_rtrim
-    ; test_escaped
-    ]
+let () = Alcotest.run "Xstringext" [test_split; test_rtrim]
diff --git a/ocaml/quicktest/quicktest_http.ml b/ocaml/quicktest/quicktest_http.ml
index 0320cf12ab3..d718a61a9e7 100644
--- a/ocaml/quicktest/quicktest_http.ml
+++ b/ocaml/quicktest/quicktest_http.ml
@@ -34,7 +34,7 @@ module Uds = struct
     with_channel_aux fd func
 
   let http_response_code d =
-    match Xapi_stdext_std.Xstringext.String.split ' ' d with
+    match String.split_on_char ' ' d with
     | _ :: code :: _ ->
         int_of_string code
     | _ ->
@@ -99,7 +99,7 @@ module Cookies = struct
       match body with
       | first_line :: _ ->
           D.warn "expected = [%s]; received = [%s]" expected first_line ;
-          Xapi_stdext_std.Xstringext.String.has_substr first_line expected
+          Astring.String.is_infix ~affix:first_line expected
       | _ ->
           false
     in
@@ -210,9 +210,7 @@ module HTML_Escaping = struct
   let bad_command_exp = "&lt;&gt;&apos;\\&quot;&amp;"
 
   let html_escaping expected cmd =
-    let check_result b =
-      Xapi_stdext_std.Xstringext.String.has_substr b expected
-    in
+    let check_result = Astring.String.is_infix ~affix:expected in
     let _, _, _, body = Uds.http_command Xapi_globs.unix_domain_socket cmd in
     match body with
     | first_line :: _ ->
diff --git a/ocaml/rrd2csv/src/rrd2csv.ml b/ocaml/rrd2csv/src/rrd2csv.ml
index 37e00f8148d..bb57fbbb8d3 100644
--- a/ocaml/rrd2csv/src/rrd2csv.ml
+++ b/ocaml/rrd2csv/src/rrd2csv.ml
@@ -149,10 +149,10 @@ module Ds_selector = struct
 
   let of_string str =
     let open Rrd in
-    let splitted = Xstringext.String.split ',' str in
+    let splitted = String.split_on_char ',' str in
     match splitted with
     | without_trailing_comma :: _ -> (
-        let splitted = Xstringext.String.split ':' without_trailing_comma in
+        let splitted = String.split_on_char ':' without_trailing_comma in
         match splitted with
         | [cf; owner; uuid; metric] ->
             {
@@ -185,14 +185,7 @@ module Ds_selector = struct
   let escape_metric s =
     let quote s = Printf.sprintf "\"%s\"" s in
     if String.contains s '"' then
-      quote
-        (Xstringext.String.map_unlikely s (function
-          | '\"' ->
-              Some "\"\""
-          | _ ->
-              None
-          )
-          )
+      quote (Xstringext.String.replace '"' ~by:{|""|} s)
     else if String.contains s ',' || String.contains s '\n' then
       quote s
     else
diff --git a/ocaml/tests/test_guest_agent.ml b/ocaml/tests/test_guest_agent.ml
index 6bc0f227c7b..94774ac10c7 100644
--- a/ocaml/tests/test_guest_agent.ml
+++ b/ocaml/tests/test_guest_agent.ml
@@ -42,9 +42,7 @@ module Networks = Generic.MakeStateless (struct
     )
 
   let construct_tree tree path =
-    let nodes =
-      Xapi_stdext_std.Xstringext.String.split_f (fun s -> s = '/') path
-    in
+    let nodes = Astring.String.cuts ~empty:false ~sep:"/" path in
     add_path_to_tree tree nodes
 
   let rec list_helper children = function
@@ -60,9 +58,7 @@ module Networks = Generic.MakeStateless (struct
     )
 
   let list (T (_root, children)) path =
-    let nodes =
-      Xapi_stdext_std.Xstringext.String.split_f (fun s -> s = '/') path
-    in
+    let nodes = Astring.String.cuts ~empty:false ~sep:"/" path in
     list_helper children nodes
 
   let transform input =
@@ -231,9 +227,7 @@ module Initial_guest_metrics = Generic.MakeStateless (struct
     )
 
   let construct_mtree mtree (path, leaf_value) =
-    let nodes =
-      Xapi_stdext_std.Xstringext.String.split_f (fun s -> s = '/') path
-    in
+    let nodes = Astring.String.cuts ~empty:false ~sep:"/" path in
     add_leaf_to_mtree nodes leaf_value mtree
 
   let rec list_helper children = function
@@ -254,9 +248,7 @@ module Initial_guest_metrics = Generic.MakeStateless (struct
     | Lf (_, _) ->
         []
     | Mt (_, children) ->
-        let nodes =
-          Xapi_stdext_std.Xstringext.String.split_f (fun s -> s = '/') path
-        in
+        let nodes = Astring.String.cuts ~empty:false ~sep:"/" path in
         list_helper children nodes
 
   let rec lookup_helper mtree = function
@@ -274,9 +266,7 @@ module Initial_guest_metrics = Generic.MakeStateless (struct
     )
 
   let lookup mtree path =
-    let nodes =
-      Xapi_stdext_std.Xstringext.String.split_f (fun s -> s = '/') path
-    in
+    let nodes = Astring.String.cuts ~empty:false ~sep:"/" path in
     lookup_helper mtree nodes
 
   let transform input =
diff --git a/ocaml/xapi-cli-server/cli_printer.ml b/ocaml/xapi-cli-server/cli_printer.ml
index 1fc1d5586fd..bb627812924 100644
--- a/ocaml/xapi-cli-server/cli_printer.ml
+++ b/ocaml/xapi-cli-server/cli_printer.ml
@@ -47,9 +47,15 @@ let rec multi_line_record r =
 
 (* Used to escape commas in --minimal mode *)
 let escape_commas x =
-  (* Escaping rules: *)
-  let rules = [(',', "\\,"); (* , -> \, *) ('\\', "\\\\") (* \ -> \\ *)] in
-  Xapi_stdext_std.Xstringext.String.escaped ~rules x
+  let replace = function
+    | ',' ->
+        Some "\\,"
+    | '\\' ->
+        Some "\\\\"
+    | _ ->
+        None
+  in
+  Xapi_stdext_std.Xstringext.String.replaced ~replace x
 
 let make_printer sock minimal =
   let buffer = ref [] in
diff --git a/ocaml/xapi/audit_log.ml b/ocaml/xapi/audit_log.ml
index 0563b2c4fe3..473df72f39e 100644
--- a/ocaml/xapi/audit_log.ml
+++ b/ocaml/xapi/audit_log.ml
@@ -102,12 +102,8 @@ let transfer_all_audit_files fd_out ?filter since =
   atransfer_try_gz ""
 
 (* map the ISO8601 timestamp format into the one in our logs *)
-let log_timestamp_of_iso8601 iso8601_timestamp =
-  let module Xstringext = Xapi_stdext_std.Xstringext in
-  let step1 = iso8601_timestamp in
-  let step2 = Xstringext.String.replace "-" "" step1 in
-  let step3 = Xstringext.String.replace "Z" "" step2 in
-  step3
+let log_timestamp_of_iso8601 iso8601 =
+  Astring.String.filter (function '-' | 'Z' -> false | _ -> true) iso8601
 
 (*
  Assume that RBAC access for the session_id already verified by xapi_http.ml
diff --git a/ocaml/xapi/authx.ml b/ocaml/xapi/authx.ml
index 87d85e40332..7236c793fb4 100644
--- a/ocaml/xapi/authx.ml
+++ b/ocaml/xapi/authx.ml
@@ -65,7 +65,7 @@ module AuthX : Auth_signature.AUTH_MODULE = struct
             | [] ->
                 raise Not_found
             | line :: lines -> (
-                let recs = Xapi_stdext_std.Xstringext.String.split ':' line in
+                let recs = String.split_on_char ':' line in
                 let username = List.nth recs 0 in
                 let uid = List.nth recs 2 in
                 match fn username uid recs with
@@ -291,9 +291,8 @@ module AuthX : Auth_signature.AUTH_MODULE = struct
     | _ ->
         raise Not_found
 
-  (*
-	In addition, there are some event hooks that auth modules implement as follows:
-*)
+  (* In addition, there are some event hooks that auth modules implement as
+     follows: *)
 
   (* unit on_enable(((string*string) list) config_params)
 
diff --git a/ocaml/xapi/config_file_sync.ml b/ocaml/xapi/config_file_sync.ml
index b765f1ceae6..cf877745290 100644
--- a/ocaml/xapi/config_file_sync.ml
+++ b/ocaml/xapi/config_file_sync.ml
@@ -15,7 +15,6 @@
 module D = Debug.Make (struct let name = "config_file_sync" end)
 
 open D
-open Xapi_stdext_std.Xstringext
 
 let superuser = "root"
 
@@ -58,7 +57,8 @@ let config_file_sync_handler (req : Http.Request.t) s _ =
   Xapi_http.with_context "Syncing dom0 config files over HTTP" req s
     (fun __context ->
       let uri =
-        String.split '/' req.Http.Request.path |> List.filter (fun x -> x <> "")
+        String.split_on_char '/' req.Http.Request.path
+        |> List.filter (fun x -> x <> "")
       in
       req.Http.Request.close <- true ;
       debug "sending headers" ;
diff --git a/ocaml/xapi/dbsync_slave.ml b/ocaml/xapi/dbsync_slave.ml
index ff325b7259e..ea5dddf8f82 100644
--- a/ocaml/xapi/dbsync_slave.ml
+++ b/ocaml/xapi/dbsync_slave.ml
@@ -17,7 +17,6 @@
 *)
 
 module Rrdd = Rrd_client.Client
-open Xapi_stdext_std.Xstringext
 module Unixext = Xapi_stdext_unix.Unixext
 module Date = Clock.Date
 open Create_misc
@@ -75,9 +74,9 @@ let get_start_time () =
     match
       Unixext.string_of_file "/proc/stat"
       |> String.trim
-      |> String.split '\n'
+      |> String.split_on_char '\n'
       |> List.find (fun s -> String.starts_with ~prefix:"btime" s)
-      |> String.split ' '
+      |> String.split_on_char ' '
     with
     | _ :: btime :: _ ->
         let boot_time = Date.of_unix_time (float_of_string btime) in
@@ -111,7 +110,7 @@ let refresh_localhost_info ~__context info =
     | None ->
         []
     | Some {capabilities; _} ->
-        String.split ' ' capabilities
+        String.split_on_char ' ' capabilities
   in
   Db.Host.set_capabilities ~__context ~self:host ~value:caps ;
   Db.Host.set_address ~__context ~self:host ~value:(get_my_ip_addr ~__context) ;
diff --git a/ocaml/xapi/extauth_plugin_ADpbis.ml b/ocaml/xapi/extauth_plugin_ADpbis.ml
index ea2dedfccc6..b57b2c91cae 100644
--- a/ocaml/xapi/extauth_plugin_ADpbis.ml
+++ b/ocaml/xapi/extauth_plugin_ADpbis.ml
@@ -18,7 +18,7 @@
 module D = Debug.Make (struct let name = "extauth_plugin_ADpbis" end)
 
 open D
-open Xapi_stdext_std.Xstringext
+module Stringext = Xapi_stdext_std.Xstringext.String
 
 let ( let@ ) = ( @@ )
 
@@ -92,6 +92,8 @@ module Lwsmd = struct
     )
 end
 
+let is_word_sep = function '(' | ')' | ' ' | '\t' | '.' -> true | _ -> false
+
 let match_error_tag (lines : string list) =
   let err_catch_list =
     [
@@ -105,8 +107,7 @@ let match_error_tag (lines : string list) =
     ]
   in
   let split_to_words str =
-    let seps = ['('; ')'; ' '; '\t'; '.'] in
-    String.split_f (fun s -> List.exists (fun sep -> sep = s) seps) str
+    Astring.String.fields ~empty:false ~is_sep:is_word_sep str
   in
   let rec has_err lines err_pattern =
     match lines with
@@ -131,9 +132,11 @@ let match_error_tag (lines : string list) =
 let extract_sid_from_group_list group_list =
   List.map
     (fun (_, v) ->
-      let v = String.replace ")" "" v in
-      let v = String.replace "sid =" "|" v in
-      let vs = String.split_f (fun c -> c = '|') v in
+      let vs =
+        Astring.String.filter (function ')' -> false | _ -> true) v
+        |> Astring.String.cuts ~empty:false ~sep:"sid ="
+        |> List.concat_map (Astring.String.cuts ~empty:false ~sep:"|")
+      in
       let sid = String.trim (List.nth vs 1) in
       debug "extract_sid_from_group_list get sid=[%s]" sid ;
       sid
@@ -166,7 +169,8 @@ module AuthADlw : Auth_signature.AUTH_MODULE = struct
     Locking_helpers.Named_mutex.create "IS_SERVER_AVAILABLE"
 
   let splitlines s =
-    String.split_f (fun c -> c = '\n') (String.replace "#012" "\n" s)
+    Astring.String.cuts ~empty:false ~sep:"#012" s
+    |> List.concat_map (Astring.String.cuts ~empty:false ~sep:"\n")
 
   let pbis_common_with_password (password : string) (pbis_cmd : string)
       (pbis_args : string list) =
@@ -238,7 +242,7 @@ module AuthADlw : Auth_signature.AUTH_MODULE = struct
       pbis_cmd ^ " " ^ List.fold_left (fun p pp -> p ^ " " ^ pp) " " pbis_args
     in
     let debug_cmd =
-      if String.has_substr debug_cmd "--password" then
+      if Astring.String.is_infix ~affix:"--password" debug_cmd then
         "(omitted for security)"
       else
         debug_cmd
@@ -348,9 +352,9 @@ module AuthADlw : Auth_signature.AUTH_MODULE = struct
         if !exited_code <> 0 then (
           error "execute '%s': exit_code=[%d] output=[%s]" debug_cmd
             !exited_code
-            (String.replace "\n" ";" !output) ;
+            (Stringext.replace '\n' ~by:";" !output) ;
           let split_to_words s =
-            String.split_f (fun c -> c = '(' || c = ')' || c = '.' || c = ' ') s
+            Astring.String.fields ~empty:false ~is_sep:is_word_sep s
           in
           let revlines =
             List.rev
@@ -416,7 +420,7 @@ module AuthADlw : Auth_signature.AUTH_MODULE = struct
           List.filter (fun l -> String.length l > 0) (splitlines !output)
         in
         let parse_line (acc, currkey) line =
-          let slices = String.split ~limit:2 ':' line in
+          let slices = Stringext.split ~limit:2 ':' line in
           debug "parse %s: currkey=[%s] line=[%s]" debug_cmd currkey line ;
           if List.length slices > 1 then (
             let key = String.trim (List.hd slices) in
@@ -619,7 +623,7 @@ module AuthADlw : Auth_signature.AUTH_MODULE = struct
     (* first, we try to authenticated user against our external user database *)
     (* pbis_common will raise an Auth_failure if external authentication fails *)
     let domain, user =
-      match String.split_f (fun c -> c = '\\') username with
+      match Astring.String.cuts ~empty:false ~sep:"\\" username with
       | [domain; user] ->
           (domain, user)
       | [user] ->
@@ -976,7 +980,8 @@ module AuthADlw : Auth_signature.AUTH_MODULE = struct
             | "" ->
                 []
             | disabled_modules_string ->
-                String.split_f (fun c -> c = ',') disabled_modules_string
+                Astring.String.cuts ~empty:false ~sep:","
+                  disabled_modules_string
           with Not_found -> []
         in
         let disabled_module_params =
@@ -1113,8 +1118,8 @@ module AuthADlw : Auth_signature.AUTH_MODULE = struct
         in
         debug "execute %s: stdout=[%s],stderr=[%s]"
           pbis_force_domain_leave_script
-          (String.replace "\n" ";" output)
-          (String.replace "\n" ";" stderr)
+          (Stringext.replace '\n' ~by:";" output)
+          (Stringext.replace '\n' ~by:";" stderr)
       with e ->
         debug "exception executing %s: %s" pbis_force_domain_leave_script
           (ExnHelper.string_of_exn e)
diff --git a/ocaml/xapi/extauth_plugin_ADwinbind.ml b/ocaml/xapi/extauth_plugin_ADwinbind.ml
index 837dc429ca2..86d0e8c25b2 100644
--- a/ocaml/xapi/extauth_plugin_ADwinbind.ml
+++ b/ocaml/xapi/extauth_plugin_ADwinbind.ml
@@ -20,7 +20,6 @@ module D = Debug.Make (struct
 end)
 
 open D
-open Xapi_stdext_std.Xstringext
 open Auth_signature
 module Scheduler = Xapi_stdext_threads_scheduler.Scheduler
 
diff --git a/ocaml/xapi/fileserver.ml b/ocaml/xapi/fileserver.ml
index 9820a55d5ce..8c83dac57f3 100644
--- a/ocaml/xapi/fileserver.ml
+++ b/ocaml/xapi/fileserver.ml
@@ -16,29 +16,17 @@
  *)
 
 open Http
-open Xapi_stdext_std.Xstringext
+module Xstringext = Xapi_stdext_std.Xstringext.String
 
 module D = Debug.Make (struct let name = "fileserver" end)
 
 open D
 
-let escape uri =
-  String.escaped
-    ~rules:
-      [
-        ('<', "&lt;")
-      ; ('>', "&gt;")
-      ; ('\'', "&apos;")
-      ; ('"', "&quot;")
-      ; ('&', "&amp;")
-      ]
-    uri
-
 let missing uri =
   "<!DOCTYPE HTML PUBLIC \"-//IETF//DTD HTML 2.0//EN\"> <html><head> \
    <title>404 Not Found</title> </head><body> <h1>Not Found</h1> <p>The \
    requested URL "
-  ^ escape uri
+  ^ Http_svr.escape_html uri
   ^ " was not found on this server.</p> <hr><address>Xapi \
      Server</address></body></html>"
 
diff --git a/ocaml/xapi/gpg.ml b/ocaml/xapi/gpg.ml
index 1dd5c8141c8..def89c7890b 100644
--- a/ocaml/xapi/gpg.ml
+++ b/ocaml/xapi/gpg.ml
@@ -13,7 +13,6 @@
  *)
 (** Wrapper around gpg *)
 
-open Xapi_stdext_std.Xstringext
 open Xapi_stdext_pervasives.Pervasiveext
 module Unixext = Xapi_stdext_unix.Unixext
 
@@ -26,7 +25,7 @@ let gpg_binary_path = "/usr/bin/gpg"
 exception InvalidSignature
 
 let parse_gpg_status status_data =
-  let lines = String.split '\n' status_data in
+  let lines = String.split_on_char '\n' status_data in
   let status_contains substr =
     List.exists (fun s -> String.starts_with ~prefix:substr s) lines
   in
@@ -42,7 +41,7 @@ let parse_gpg_status status_data =
     let validsigline =
       List.find (fun s -> String.starts_with ~prefix:validsig s) lines
     in
-    match String.split ' ' validsigline with
+    match String.split_on_char ' ' validsigline with
     | _ :: _ :: fingerprint :: _ ->
         Some fingerprint
     | _ ->
diff --git a/ocaml/xapi/import.ml b/ocaml/xapi/import.ml
index 04306cff5f6..c854ed1807d 100644
--- a/ocaml/xapi/import.ml
+++ b/ocaml/xapi/import.ml
@@ -19,7 +19,6 @@ module D = Debug.Make (struct let name = "import" end)
 
 open D
 module Listext = Xapi_stdext_std.Listext
-module Xstringext = Xapi_stdext_std.Xstringext
 module Unixext = Xapi_stdext_unix.Unixext
 open Http
 open Importexport
diff --git a/ocaml/xapi/map_check.ml b/ocaml/xapi/map_check.ml
index 0cb2d97e37f..3737b3f00bf 100644
--- a/ocaml/xapi/map_check.ml
+++ b/ocaml/xapi/map_check.ml
@@ -104,7 +104,7 @@ let assert_value ~field ~key ~attr ~value =
   )
   | EnumSet range ->
       (* enumset is a comma-separated string *)
-      let vs = Xapi_stdext_std.Xstringext.String.split ',' value in
+      let vs = String.split_on_char ',' value in
       List.fold_right
         (fun v acc ->
           match mem v range with
@@ -113,7 +113,7 @@ let assert_value ~field ~key ~attr ~value =
           | Some v ->
               if acc = "" then
                 v
-              else if Xapi_stdext_std.Xstringext.String.has_substr acc v then
+              else if Astring.String.is_infix ~affix:v acc then
                 err value
               else
                 v ^ "," ^ acc
diff --git a/ocaml/xapi/nm.ml b/ocaml/xapi/nm.ml
index fa86a6f08e7..4df9bf233f9 100644
--- a/ocaml/xapi/nm.ml
+++ b/ocaml/xapi/nm.ml
@@ -14,7 +14,7 @@
 module D = Debug.Make (struct let name = "nm" end)
 
 open D
-open Xapi_stdext_std.Xstringext
+module Stringext = Xapi_stdext_std.Xstringext.String
 module Listext = Xapi_stdext_std.Listext.List
 open Xapi_database.Db_filter_types
 open Network
@@ -216,7 +216,7 @@ let create_bond ~__context bond mtu persistent =
         List.filter_map
           (fun (k, v) ->
             if String.starts_with ~prefix:"bond-" k then
-              Some (String.sub_to_end k 5, v)
+              Some (Stringext.sub_to_end k 5, v)
             else
               None
           )
@@ -470,7 +470,7 @@ let determine_static_routes net_rc =
   if List.mem_assoc "static-routes" net_rc.API.network_other_config then
     try
       let routes =
-        String.split ','
+        String.split_on_char ','
           (List.assoc "static-routes" net_rc.API.network_other_config)
       in
       List.map
diff --git a/ocaml/xapi/pciops.ml b/ocaml/xapi/pciops.ml
index 2126185474e..46cfdf377cc 100644
--- a/ocaml/xapi/pciops.ml
+++ b/ocaml/xapi/pciops.ml
@@ -126,7 +126,8 @@ let _unhide_pci ~__context pci =
       Printf.sprintf "(%s)" (Db.PCI.get_pci_id ~__context ~self:pci)
     in
     let new_value =
-      Xapi_stdext_std.Xstringext.String.replace bdf_paren "" raw_value
+      Astring.String.cuts ~empty:false ~sep:bdf_paren raw_value
+      |> String.concat ""
     in
     let cmd =
       match new_value with
diff --git a/ocaml/xapi/pvs_proxy_control.ml b/ocaml/xapi/pvs_proxy_control.ml
index 8597166fe7b..8a64cb2c051 100644
--- a/ocaml/xapi/pvs_proxy_control.ml
+++ b/ocaml/xapi/pvs_proxy_control.ml
@@ -21,7 +21,9 @@ open D
 let proxy_port_name vif =
   (* Interface names in Linux are at most 15 characters. We derive a
      name from the MAC address to ensure uniqueness, and make it fit. *)
-  let mac = Xapi_stdext_std.Xstringext.String.replace ":" "" vif.API.vIF_MAC in
+  let mac =
+    Astring.String.filter (function ':' -> false | _ -> true) vif.API.vIF_MAC
+  in
   Printf.sprintf "pvs%s" mac
 
 (** [proxies] returns all currently attached proxies *)
diff --git a/ocaml/xapi/rbac_audit.ml b/ocaml/xapi/rbac_audit.ml
index bbc5a7a6fc9..7f86ba31030 100644
--- a/ocaml/xapi/rbac_audit.ml
+++ b/ocaml/xapi/rbac_audit.ml
@@ -362,8 +362,7 @@ and
 
 let has_to_audit action =
   let has_side_effect action =
-    not (Xapi_stdext_std.Xstringext.String.has_substr action ".get")
-    (* TODO: a bit slow? *)
+    not (Astring.String.is_infix ~affix:".get" action)
   in
   (!Xapi_globs.log_getter || has_side_effect action)
   && not
@@ -454,9 +453,9 @@ let audit_line_of __context session_id allowed_denied ok_error result_error
          ?sexpr_of_args action permission
       )
   in
-  let line = Xapi_stdext_std.Xstringext.String.replace "\n" " " _line in
+  let line = Xapi_stdext_std.Xstringext.String.replace '\n' ~by:" " _line in
   (* no \n in line *)
-  let line = Xapi_stdext_std.Xstringext.String.replace "\r" " " line in
+  let line = Xapi_stdext_std.Xstringext.String.replace '\r' ~by:" " line in
   (* no \r in line *)
   let audit_line = append_line "%s" line in
   (*D.debug "line=%s, audit_line=%s" line audit_line;*)
@@ -471,7 +470,7 @@ let allowed_pre_fn ~__context ~action ?args () =
     if
       has_to_audit action
       (* for now, we only cache arg results for destroy actions *)
-      && Xapi_stdext_std.Xstringext.String.has_substr action ".destroy"
+      && Astring.String.is_infix ~affix:".destroy" action
     then
       let args' = add_dummy_args __context action args in
       Some (sexpr_of_parameters __context action args')
diff --git a/ocaml/xapi/sm.ml b/ocaml/xapi/sm.ml
index 1d198cf3f98..532f6fb44b1 100644
--- a/ocaml/xapi/sm.ml
+++ b/ocaml/xapi/sm.ml
@@ -15,7 +15,6 @@
  * @group Storage
 *)
 
-open Xapi_stdext_std.Xstringext
 open Smint
 open Printf
 
diff --git a/ocaml/xapi/storage_mux.ml b/ocaml/xapi/storage_mux.ml
index 0427f76ca54..f2091c896d0 100644
--- a/ocaml/xapi/storage_mux.ml
+++ b/ocaml/xapi/storage_mux.ml
@@ -97,7 +97,8 @@ module Mux = struct
 
     let m = Mutex.create ()
 
-    let filename_of dp = Xapi_stdext_std.Xstringext.String.replace "/" "-" dp
+    let filename_of dp =
+      Xapi_stdext_std.Xstringext.String.replace '/' ~by:"-" dp
 
     let write dp info =
       let filename = filename_of dp in
@@ -780,9 +781,9 @@ module Mux = struct
     (* Assume it has either the format:
        SR/VDI -- for a particular SR and VDI
        content_id -- for a particular content *)
-    let open Xapi_stdext_std.Xstringext in
+    let split = Xapi_stdext_std.Xstringext.String.split in
     with_dbg ~name:"get_by_name" ~dbg @@ fun di ->
-    match List.filter (fun x -> x <> "") (String.split ~limit:2 '/' name) with
+    match List.filter (fun x -> x <> "") (split ~limit:2 '/' name) with
     | [sr; name] ->
         let sr = Storage_interface.Sr.of_string sr in
         let module C = StorageAPI (Idl.Exn.GenClient (struct
diff --git a/ocaml/xapi/vgpuops.ml b/ocaml/xapi/vgpuops.ml
index e47b0896ab3..248ec2839a1 100644
--- a/ocaml/xapi/vgpuops.ml
+++ b/ocaml/xapi/vgpuops.ml
@@ -15,7 +15,6 @@ module D = Debug.Make (struct let name = "vgpuops" end)
 
 open D
 module Listext = Xapi_stdext_std.Listext.List
-open Xapi_stdext_std.Xstringext
 
 type vgpu_t = {
     vgpu_ref: API.ref_VGPU
diff --git a/ocaml/xapi/vhd_tool_wrapper.ml b/ocaml/xapi/vhd_tool_wrapper.ml
index f3f791fe251..e8c051c0ef7 100644
--- a/ocaml/xapi/vhd_tool_wrapper.ml
+++ b/ocaml/xapi/vhd_tool_wrapper.ml
@@ -18,7 +18,6 @@
 module D = Debug.Make (struct let name = "vhd_tool_wrapper" end)
 
 open D
-open Xapi_stdext_std.Xstringext
 
 (* .vhds on XenServer are sometimes found via /dev/mapper *)
 let vhd_search_path = "/dev/mapper:."
@@ -125,7 +124,7 @@ let find_backend_device path =
     let link =
       Unix.readlink (Printf.sprintf "/sys/dev/block/%d:%d/device" major minor)
     in
-    match List.rev (String.split '/' link) with
+    match List.rev (String.split_on_char '/' link) with
     | id :: "xen" :: "devices" :: _
       when Astring.String.is_prefix ~affix:"vbd-" id ->
         let id = int_of_string (String.sub id 4 (String.length id - 4)) in
@@ -135,7 +134,7 @@ let find_backend_device path =
               xs.Xs.read (Printf.sprintf "device/vbd/%d/backend" id)
             in
             let params = xs.Xs.read (Printf.sprintf "%s/params" backend) in
-            match String.split '/' backend with
+            match String.split_on_char '/' backend with
             | "local" :: "domain" :: bedomid :: _ ->
                 if not (self = bedomid) then
                   Helpers.internal_error
diff --git a/ocaml/xapi/wlb_reports.ml b/ocaml/xapi/wlb_reports.ml
index e466bf959f5..8de0e80b298 100644
--- a/ocaml/xapi/wlb_reports.ml
+++ b/ocaml/xapi/wlb_reports.ml
@@ -92,7 +92,6 @@
 
 open Printf
 open Http
-open Xapi_stdext_std.Xstringext
 
 module D = Debug.Make (struct let name = "wlb_reports" end)
 
diff --git a/ocaml/xapi/workload_balancing.ml b/ocaml/xapi/workload_balancing.ml
index 7108032dbf7..136aa2c8985 100644
--- a/ocaml/xapi/workload_balancing.ml
+++ b/ocaml/xapi/workload_balancing.ml
@@ -16,7 +16,6 @@
 *)
 
 open Printf
-open Xapi_stdext_std.Xstringext
 
 module D = Debug.Make (struct let name = "workload_balancing" end)
 
@@ -75,24 +74,12 @@ let raise_internal_error args =
   raise (Api_errors.Server_error (Api_errors.wlb_internal_error, args))
 
 let split_host_port url =
-  try
-    if url.[0] = '[' then (
-      (* IPv6 *)
-      let host_end = String.rindex url ']' in
-      if url.[host_end + 1] <> ':' then
-        raise_url_invalid url ;
-      let host = String.sub url 1 (host_end - 1) in
-      let port =
-        String.sub url (host_end + 2) (String.length url - host_end - 2)
-      in
-      (host, int_of_string port)
-    ) else
-      match String.split_f (fun a -> a = ':') url with
-      | [host; port] ->
-          (host, int_of_string port)
-      | _ ->
-          raise_url_invalid url
-  with _ -> raise_url_invalid url
+  let uri = Uri.of_string ("//" ^ url) in
+  match (Uri.host uri, Uri.port uri) with
+  | None, _ | _, None ->
+      raise_url_invalid url
+  | Some host, Some port ->
+      (host, port)
 
 let wlb_host_port ~__context =
   let pool = Helpers.get_pool ~__context in
diff --git a/ocaml/xapi/xapi_dr_task.ml b/ocaml/xapi/xapi_dr_task.ml
index de7d15e0523..56626ea1368 100644
--- a/ocaml/xapi/xapi_dr_task.ml
+++ b/ocaml/xapi/xapi_dr_task.ml
@@ -13,7 +13,6 @@
  *)
 
 open Client
-open Xapi_stdext_std.Xstringext
 
 module D = Debug.Make (struct let name = "xapi_dr_task" end)
 
diff --git a/ocaml/xapi/xapi_globs.ml b/ocaml/xapi/xapi_globs.ml
index 161273c83f9..8a062edbf29 100644
--- a/ocaml/xapi/xapi_globs.ml
+++ b/ocaml/xapi/xapi_globs.ml
@@ -14,8 +14,6 @@
 
 (** A central location for settings related to xapi *)
 
-module String_plain = String (* For when we don't want the Xstringext version *)
-open Xapi_stdext_std.Xstringext
 module StringSet = Set.Make (String)
 
 module D = Debug.Make (struct let name = "xapi_globs" end)
@@ -1366,12 +1364,12 @@ let citrix_patch_key =
 
 let trusted_patch_key = ref citrix_patch_key
 
+let fields_of = Astring.(String.fields ~empty:false ~is_sep:Char.Ascii.is_white)
+
 let gen_list_option name desc of_string string_of opt =
   let parse s =
     opt := [] ;
-    try
-      String.split_f String.isspace s
-      |> List.iter (fun x -> opt := of_string x :: !opt)
+    try fields_of s |> List.iter (fun x -> opt := of_string x :: !opt)
     with e ->
       D.error "Unable to parse %s=%s (expected space-separated list) error: %s"
         name s (Printexc.to_string e)
@@ -1466,7 +1464,7 @@ let other_options =
       (fun s -> s)
       disable_dbsync_for
   ; ( "xenopsd-queues"
-    , Arg.String (fun x -> xenopsd_queues := String.split ',' x)
+    , Arg.String (fun x -> xenopsd_queues := String.split_on_char ',' x)
     , (fun () -> String.concat "," !xenopsd_queues)
     , "list of xenopsd instances to manage"
     )
@@ -1553,7 +1551,8 @@ let other_options =
   ; ( "nvidia_multi_vgpu_enabled_driver_versions"
     , Arg.String
         (fun x ->
-          nvidia_multi_vgpu_enabled_driver_versions := String.split ',' x
+          nvidia_multi_vgpu_enabled_driver_versions :=
+            String.split_on_char ',' x
         )
     , (fun () -> String.concat "," !nvidia_multi_vgpu_enabled_driver_versions)
     , "list of nvidia host driver versions with multiple vGPU supported.\n\
diff --git a/ocaml/xapi/xapi_host_crashdump.ml b/ocaml/xapi/xapi_host_crashdump.ml
index b7e9eedd74e..f9c66de62c8 100644
--- a/ocaml/xapi/xapi_host_crashdump.ml
+++ b/ocaml/xapi/xapi_host_crashdump.ml
@@ -16,7 +16,6 @@
 *)
 
 module Listext = Xapi_stdext_std.Listext
-module Xstringext = Xapi_stdext_std.Xstringext
 module Date = Clock.Date
 open Xapi_stdext_pervasives.Pervasiveext
 open Xapi_support
@@ -50,6 +49,8 @@ let delete_crashdump_dir filename =
       (ExnHelper.string_of_exn e) ;
     raise e
 
+let fields_of = Astring.(String.fields ~empty:false ~is_sep:Char.Ascii.is_white)
+
 (* Called once on host boot to resync the crash directory with the database *)
 let resynchronise ~__context ~host =
   debug "Xapi_host_crashdump.resynchronise" ;
@@ -103,9 +104,7 @@ let resynchronise ~__context ~host =
       debug "Adding record corresponding to new crashdump %s" filename ;
       let cmd = Printf.sprintf "%s --bytes -s %s/%s" du crash_dir filename in
       let size =
-        match
-          Xstringext.String.(split_f isspace (Helpers.get_process_output cmd))
-        with
+        match fields_of (Helpers.get_process_output cmd) with
         | size :: _ ->
             Int64.of_string size
         | _ ->
diff --git a/ocaml/xapi/xapi_network.ml b/ocaml/xapi/xapi_network.ml
index 73a59fd698c..dd937ad5784 100644
--- a/ocaml/xapi/xapi_network.ml
+++ b/ocaml/xapi/xapi_network.ml
@@ -14,7 +14,6 @@
 let with_lock = Xapi_stdext_threads.Threadext.Mutex.execute
 
 open Client
-open Xapi_stdext_std.Xstringext
 
 module D = Debug.Make (struct let name = "xapi_network" end)
 
diff --git a/ocaml/xapi/xapi_pif.ml b/ocaml/xapi/xapi_pif.ml
index 881c51091fb..ccebf243816 100644
--- a/ocaml/xapi/xapi_pif.ml
+++ b/ocaml/xapi/xapi_pif.ml
@@ -19,7 +19,6 @@ module L = Debug.Make (struct let name = "license" end)
 
 open Xapi_database.Db_filter_types
 module Listext = Xapi_stdext_std.Listext.List
-open Xapi_stdext_std.Xstringext
 module Date = Clock.Date
 open Network
 
@@ -216,7 +215,9 @@ let refresh_all ~__context ~host =
     pifs
 
 let read_bridges_from_inventory () =
-  try String.split ' ' (Xapi_inventory.lookup Xapi_inventory._current_interfaces)
+  try
+    String.split_on_char ' '
+      (Xapi_inventory.lookup Xapi_inventory._current_interfaces)
   with _ -> []
 
 (* Ensure the PIF is not a bond slave. *)
@@ -707,6 +708,8 @@ let forget ~__context ~self =
 
 let scan_m = Mutex.create ()
 
+let fields_of = Astring.(String.fields ~empty:false ~is_sep:Char.Ascii.is_white)
+
 let scan ~__context ~host =
   let dbg = Context.string_of_task __context in
   refresh_all ~__context ~host ;
@@ -716,15 +719,15 @@ let scan ~__context ~host =
         let output, _ =
           Forkhelpers.execute_command_get_output !Xapi_globs.non_managed_pifs []
         in
-        let dsplit = String.split '\n' output in
+        let dsplit = String.split_on_char '\n' output in
         match dsplit with
         | [] | [""] | "" :: "" :: _ ->
             debug "No boot from SAN interface found" ;
             ([], [])
         | m :: u :: _ ->
-            (String.split_f String.isspace m, String.split_f String.isspace u)
+            (fields_of m, fields_of u)
         | m :: _ ->
-            (String.split_f String.isspace m, [])
+            (fields_of m, [])
       with e ->
         warn "Error when executing script %s: %s; ignoring"
           !Xapi_globs.non_managed_pifs
diff --git a/ocaml/xapi/xapi_pool.ml b/ocaml/xapi/xapi_pool.ml
index 752d822135f..89e656ec8e2 100644
--- a/ocaml/xapi/xapi_pool.ml
+++ b/ocaml/xapi/xapi_pool.ml
@@ -16,7 +16,6 @@ open Client
 module Date = Clock.Date
 module Listext = Xapi_stdext_std.Listext
 module Unixext = Xapi_stdext_unix.Unixext
-module Xstringext = Xapi_stdext_std.Xstringext
 
 module Pkgs = (val Pkg_mgr.get_pkg_mgr)
 
diff --git a/ocaml/xapi/xapi_pool_update.ml b/ocaml/xapi/xapi_pool_update.ml
index 6aa1ea0fd71..a0b0534628a 100644
--- a/ocaml/xapi/xapi_pool_update.ml
+++ b/ocaml/xapi/xapi_pool_update.ml
@@ -12,7 +12,7 @@
  * GNU Lesser General Public License for more details.
  *)
 
-open Xapi_stdext_std.Xstringext
+module Stringext = Xapi_stdext_std.Xstringext.String
 module Unixext = Xapi_stdext_unix.Unixext
 open Http
 open Helpers
@@ -376,7 +376,7 @@ let parse_update_info xml =
           | "" ->
               []
           | s ->
-              List.map guidance_from_string (String.split ',' s)
+              List.map guidance_from_string (String.split_on_char ',' s)
         with _ -> []
       in
       let enforce_homogeneity =
@@ -770,9 +770,10 @@ let resync_host ~__context ~host =
 let path_and_host_from_uri uri =
   (* remove any dodgy use of "." or ".." NB we don't prevent the use of symlinks *)
   let host_and_path =
-    String.sub_to_end uri (String.length Constants.get_pool_update_download_uri)
+    Stringext.sub_to_end uri
+      (String.length Constants.get_pool_update_download_uri)
   in
-  match String.split ~limit:2 '/' host_and_path with
+  match Stringext.split ~limit:2 '/' host_and_path with
   | [host; untrusted_path] ->
       let resolved_path =
         untrusted_path
diff --git a/ocaml/xapi/xapi_secret.ml b/ocaml/xapi/xapi_secret.ml
index 57492840ead..9ca6eb88276 100644
--- a/ocaml/xapi/xapi_secret.ml
+++ b/ocaml/xapi/xapi_secret.ml
@@ -15,8 +15,6 @@
  * @group XenAPI functions
 *)
 
-open Xapi_stdext_std.Xstringext
-
 module D = Debug.Make (struct let name = "xapi_secret" end)
 
 open D
diff --git a/ocaml/xapi/xapi_vbd_helpers.ml b/ocaml/xapi/xapi_vbd_helpers.ml
index 07d6b012da2..7fe8f32aa50 100644
--- a/ocaml/xapi/xapi_vbd_helpers.ml
+++ b/ocaml/xapi/xapi_vbd_helpers.ml
@@ -15,7 +15,6 @@
  * @group Storage
 *)
 
-open Xapi_stdext_std.Xstringext
 module Listext = Xapi_stdext_std.Listext
 module Date = Clock.Date
 
diff --git a/ocaml/xapi/xapi_vif_helpers.ml b/ocaml/xapi/xapi_vif_helpers.ml
index 37de1b77770..9bc87f3a553 100644
--- a/ocaml/xapi/xapi_vif_helpers.ml
+++ b/ocaml/xapi/xapi_vif_helpers.ml
@@ -12,8 +12,6 @@
  * GNU Lesser General Public License for more details.
  *)
 
-open Xapi_stdext_std.Xstringext
-
 module D = Debug.Make (struct let name = "xapi_vif_helpers" end)
 
 open D
diff --git a/ocaml/xapi/xapi_vusb_helpers.ml b/ocaml/xapi/xapi_vusb_helpers.ml
index 19298735a06..0cf59fa5167 100644
--- a/ocaml/xapi/xapi_vusb_helpers.ml
+++ b/ocaml/xapi/xapi_vusb_helpers.ml
@@ -12,8 +12,6 @@
  * GNU Lesser General Public License for more details.
  *)
 
-open Xapi_stdext_std.Xstringext
-
 module D = Debug.Make (struct let name = "xapi_vusb_helpers" end)
 
 (**************************************************************************************)
diff --git a/ocaml/xapi/xapi_xenops.ml b/ocaml/xapi/xapi_xenops.ml
index 0ea29ea4cf7..a39a9983c86 100644
--- a/ocaml/xapi/xapi_xenops.ml
+++ b/ocaml/xapi/xapi_xenops.ml
@@ -17,9 +17,9 @@ module D = Debug.Make (struct let name = "xenops" end)
 open D
 module StringSet = Set.Make (String)
 open Network
-open Xapi_stdext_std.Xstringext
 module Date = Clock.Date
 module Listext = Xapi_stdext_std.Listext.List
+module Stringext = Xapi_stdext_std.Xstringext.String
 
 let with_lock = Xapi_stdext_threads.Threadext.Mutex.execute
 
@@ -129,7 +129,7 @@ let disk_of_vdi ~__context ~self =
 
 let vdi_of_disk ~__context x =
   let@ __context = Context.with_tracing ~__context __FUNCTION__ in
-  match String.split ~limit:2 '/' x with
+  match Stringext.split ~limit:2 '/' x with
   | [sr_uuid; location] -> (
       let open Xapi_database.Db_filter_types in
       let sr = Db.SR.get_by_uuid ~__context ~uuid:sr_uuid in
@@ -1182,8 +1182,8 @@ module MD = struct
       let affinity =
         try
           List.map
-            (fun x -> List.map int_of_string (String.split ',' x))
-            (String.split ';' (List.assoc "mask" vm.API.vM_VCPUs_params))
+            (fun x -> List.map int_of_string (String.split_on_char ',' x))
+            (String.split_on_char ';' (List.assoc "mask" vm.API.vM_VCPUs_params))
         with _ -> []
       in
       let localhost = Helpers.get_localhost ~__context in
@@ -1193,7 +1193,9 @@ module MD = struct
       let host_cpu_mask =
         try
           List.map int_of_string
-            (String.split ',' (List.assoc "mask" host_guest_VCPUs_params))
+            (String.split_on_char ','
+               (List.assoc "mask" host_guest_VCPUs_params)
+            )
         with _ -> []
       in
       let affinity =
@@ -1981,7 +1983,9 @@ let update_vm_internal ~__context ~id ~self ~previous ~info ~localhost =
               String.sub path (String.length dir)
                 (String.length path - String.length dir)
             in
-            match List.filter (fun x -> x <> "") (String.split '/' rest) with
+            match
+              List.filter (fun x -> x <> "") (String.split_on_char '/' rest)
+            with
             | x :: _ ->
                 Some x
             | _ ->
diff --git a/ocaml/xapi/xha_interface.ml b/ocaml/xapi/xha_interface.ml
index 82ac381519d..dee90350575 100644
--- a/ocaml/xapi/xha_interface.ml
+++ b/ocaml/xapi/xha_interface.ml
@@ -12,7 +12,6 @@
  * GNU Lesser General Public License for more details.
  *)
 open API
-open Xapi_stdext_std.Xstringext
 
 let hashtbl_of_list xs =
   let tbl = Hashtbl.create (List.length xs) in
@@ -350,7 +349,10 @@ module LiveSetInformation = struct
             | Some u ->
                 u
           in
-          let set f x = List.map f (String.split_f String.isspace x) in
+          let fields_of =
+            Astring.(String.fields ~empty:false ~is_sep:Char.Ascii.is_white)
+          in
+          let set f x = List.map f (fields_of x) in
           Some
             {
               id= uuid (find "HostID")
diff --git a/ocaml/xcp-rrdd/bin/rrdp-iostat/rrdp_iostat.ml b/ocaml/xcp-rrdd/bin/rrdp-iostat/rrdp_iostat.ml
index 6141090eae7..cad87fc6614 100644
--- a/ocaml/xcp-rrdd/bin/rrdp-iostat/rrdp_iostat.ml
+++ b/ocaml/xcp-rrdd/bin/rrdp-iostat/rrdp_iostat.ml
@@ -437,7 +437,7 @@ let exec_tap_ctl_list () : ((string * string) * int) list =
 let minor_of_tapdev_unsafe tapdev =
   int_of_string
     (Unixext.file_lines_fold
-       (fun acc l -> acc ^ List.nth (Xstringext.String.split ':' l) 1)
+       (fun acc l -> acc ^ List.nth (String.split_on_char ':' l) 1)
        ""
        ("/sys/block/" ^ tapdev ^ "/dev")
     )
diff --git a/ocaml/xcp-rrdd/bin/rrdp-squeezed/rrdp_squeezed.ml b/ocaml/xcp-rrdd/bin/rrdp-squeezed/rrdp_squeezed.ml
index df49dca259f..55a190b8988 100644
--- a/ocaml/xcp-rrdd/bin/rrdp-squeezed/rrdp_squeezed.ml
+++ b/ocaml/xcp-rrdd/bin/rrdp-squeezed/rrdp_squeezed.ml
@@ -12,7 +12,6 @@
  * GNU Lesser General Public License for more details.
  *)
 
-open Xapi_stdext_std
 open Rrdd_plugin
 
 module Process = Process (struct let name = "xcp-rrdd-squeezed" end)
@@ -81,7 +80,7 @@ module MemoryActions = struct
             path domid ;
           current_memory_values := IntMap.remove domid !current_memory_values
     in
-    match List.filter (fun x -> x <> "") (Xstringext.String.split '/' path) with
+    match List.filter (fun x -> x <> "") (String.split_on_char '/' path) with
     | ["local"; "domain"; domid; "memory"; "dynamic-max"] ->
         read_new_value domid current_dynamic_max_values
     | ["local"; "domain"; domid; "memory"; "dynamic-min"] ->
diff --git a/ocaml/xe-cli/dune b/ocaml/xe-cli/dune
index b61ec3cde63..3aed1017987 100644
--- a/ocaml/xe-cli/dune
+++ b/ocaml/xe-cli/dune
@@ -17,7 +17,6 @@
     xapi-backtrace
     xapi-cli-protocol
     xapi-stdext-pervasives
-    xapi-stdext-std
     xapi-stdext-unix
   )
 )
diff --git a/ocaml/xe-cli/newcli.ml b/ocaml/xe-cli/newcli.ml
index 60ecce2a47d..0b18d57dd18 100644
--- a/ocaml/xe-cli/newcli.ml
+++ b/ocaml/xe-cli/newcli.ml
@@ -13,7 +13,6 @@
  *)
 (* New cli talking to the in-server cli interface *)
 
-open Xapi_stdext_std.Xstringext
 open Xapi_stdext_pervasives
 open Cli_protocol
 
@@ -306,7 +305,7 @@ let parse_args =
             || (extra_args.[!i] = ',' && extra_args.[!i - 1] <> '\\')
           then (
           let seg = String.sub extra_args !pos (!i - !pos) in
-          l := String.filter_chars seg (( <> ) '\\') :: !l ;
+          l := Astring.String.filter (( <> ) '\\') seg :: !l ;
           incr i ;
           pos := !i
         ) else
@@ -389,7 +388,7 @@ let with_open_channels f =
   match result with Ok r -> r | Error e -> raise e
 
 let http_response_code x =
-  match String.split ' ' x with
+  match String.split_on_char ' ' x with
   | _ :: code :: _ ->
       int_of_string code
   | _ ->