diff --git a/lib/pp_diff.ml b/lib/pp_diff.ml
index d10e7390a99d8..a1888974aa71f 100644
--- a/lib/pp_diff.ml
+++ b/lib/pp_diff.ml
@@ -126,13 +126,20 @@ let diff_strs old_strs new_strs =
   let diffs = List.rev (StringDiff.diff old_strs new_strs) in
   shorten_diff_span `Removed (shorten_diff_span `Added diffs);;
 
-(* to be set to Proof_diffs.tokenize_string to allow a forward reference to the lexer *)
-let tokenize_string = ref (fun (s : string) -> [s])
+(* Default string tokenizer.  Makes each character a separate strin.
+Whitespace is not ignored.  Doesn't handle UTF-8 differences well. *)
+let def_tokenize_string s =
+  let limit = (String.length s) - 1 in
+  let strs : string list ref = ref [] in
+  for i = 0 to limit do
+    strs := (String.make 1 s.[i]) :: !strs
+  done;
+  List.rev !strs
 
 (* get the Myers diff of 2 strings *)
-let diff_str old_str new_str =
-  let old_toks = Array.of_list (!tokenize_string old_str)
-  and new_toks = Array.of_list (!tokenize_string new_str) in
+let diff_str ?(tokenize_string=def_tokenize_string) old_str new_str =
+  let old_toks = Array.of_list (tokenize_string old_str)
+  and new_toks = Array.of_list (tokenize_string new_str) in
   diff_strs old_toks new_toks;;
 
 let get_dinfo = function
@@ -277,18 +284,18 @@ let add_diff_tags which pp diffs  =
     raise (Diff_Failure "left-over diff info at end of Pp.t, should be impossible");
   if has_added || has_removed then wrap_in_bg diff_tag rv else rv;;
 
-let diff_pp o_pp n_pp =
+let diff_pp ?(tokenize_string=def_tokenize_string) o_pp n_pp =
   let open Pp in
   let o_str = string_of_ppcmds o_pp in
   let n_str = string_of_ppcmds n_pp in
-  let diffs = diff_str o_str n_str in
+  let diffs = diff_str ~tokenize_string o_str n_str in
   (add_diff_tags `Removed o_pp diffs, add_diff_tags `Added n_pp diffs);;
 
-let diff_pp_combined ?(show_removed=false) o_pp n_pp =
+let diff_pp_combined ?(tokenize_string=def_tokenize_string) ?(show_removed=false) o_pp n_pp =
   let open Pp in
   let o_str = string_of_ppcmds o_pp in
   let n_str = string_of_ppcmds n_pp in
-  let diffs = diff_str o_str n_str in
+  let diffs = diff_str ~tokenize_string o_str n_str in
   let (_, has_removed) = has_changes diffs in
   let added = add_diff_tags `Added n_pp diffs in
   if show_removed && has_removed then
diff --git a/lib/pp_diff.mli b/lib/pp_diff.mli
index 94b00fed6a35b..03468271d2ebd 100644
--- a/lib/pp_diff.mli
+++ b/lib/pp_diff.mli
@@ -33,19 +33,16 @@ Limitations/Possible enhancements:
 
 (** Compute the diff between two Pp.t structures and return
 versions of each with diffs highlighted as (old, new) *)
-val diff_pp : Pp.t -> Pp.t -> Pp.t * Pp.t
+val diff_pp : ?tokenize_string:(string -> string list) -> Pp.t -> Pp.t -> Pp.t * Pp.t
 
 (** Compute the diff between two Pp.t structures and return
 a highlighted Pp.t.  If [show_removed] is true, show separate lines for
 removals and additions, otherwise only show additions *)
-val diff_pp_combined : ?show_removed:bool -> Pp.t -> Pp.t -> Pp.t
+val diff_pp_combined : ?tokenize_string:(string -> string list) -> ?show_removed:bool -> Pp.t -> Pp.t -> Pp.t
 
 (** Raised if the diff fails *)
 exception Diff_Failure of string
 
-(* for dependency injection to allow calling the lexer *)
-val tokenize_string : (string -> string list) ref
-
 module StringDiff :
 sig
   type elem = String.t
@@ -69,7 +66,7 @@ If the strings are not lexable, this routine will raise Diff_Failure.
 Therefore you should catch any exceptions.  The workaround for now is for the
 caller to tokenize the strings itself and then call diff_strs.
 *)
-val diff_str : string -> string -> StringDiff.elem Diff2.edit list
+val diff_str : ?tokenize_string:(string -> string list) -> string -> string -> StringDiff.elem Diff2.edit list
 
 (** Compute the differences between 2 lists of strings, treating the strings
 in the lists as indivisible units.
diff --git a/printing/proof_diffs.ml b/printing/proof_diffs.ml
index efffe58018e6c..106fe3db3abb7 100644
--- a/printing/proof_diffs.ml
+++ b/printing/proof_diffs.ml
@@ -89,7 +89,6 @@ let cprintf s = cfprintf !log_out_ch s
 
 module StringMap = Map.Make(String);;
 
-(* placed here so that pp_diff.ml can access the lexer through dependency injection *)
 let tokenize_string s =
   (* todo: cLexer changes buff as it proceeds.  Seems like that should be saved, too.
   But I don't understand how it's used--it looks like things get appended to it but
@@ -112,8 +111,6 @@ let tokenize_string s =
     CLexer.set_lexer_state st;
     raise (Diff_Failure "Input string is not lexable");;
 
-let _ = Pp_diff.tokenize_string := tokenize_string
-
 
 type hyp_info = {
   idents: string list;
@@ -153,7 +150,7 @@ let diff_hyps o_line_idents o_map n_line_idents n_map =
     let (o_line, o_pp) = setup old_ids o_map in
     let (n_line, n_pp) = setup new_ids n_map in
 
-    let hyp_diffs = diff_str o_line n_line in
+    let hyp_diffs = diff_str ~tokenize_string o_line n_line in
     let (has_added, has_removed) = has_changes hyp_diffs in
     if show_removed () && has_removed then begin
       let o_entry = StringMap.find (List.hd old_ids) o_map in
@@ -303,7 +300,7 @@ let diff_goal_info o_info n_info =
     let (o_line_idents, o_hyp_map, o_concl_pp) = o_info in
     let (n_line_idents, n_hyp_map, n_concl_pp) = n_info in
     let show_removed = Some (show_removed ()) in
-    let concl_pp = diff_pp_combined ?show_removed o_concl_pp n_concl_pp in
+    let concl_pp = diff_pp_combined ~tokenize_string ?show_removed o_concl_pp n_concl_pp in
 
     let hyp_diffs_list = diff_hyps o_line_idents o_hyp_map n_line_idents n_hyp_map in
     (hyp_diffs_list, concl_pp)
diff --git a/printing/proof_diffs.mli b/printing/proof_diffs.mli
index 481b664d8aeee..0d3b5821e5113 100644
--- a/printing/proof_diffs.mli
+++ b/printing/proof_diffs.mli
@@ -43,6 +43,9 @@ the first argument set to None, which will skip the diff.
 *)
 val diff_goals : ?prev_gs:(goal sigma) -> goal sigma option -> Pp.t
 
+(** Convert a string to a list of token strings using the lexer *)
+val tokenize_string : string -> string list
+
 (* Exposed for unit test, don't use these otherwise *)
 (* output channel for the test log file *)
 val log_out_ch : out_channel ref
diff --git a/test-suite/unit-tests/printing/proof_diffs_test.ml b/test-suite/unit-tests/printing/proof_diffs_test.ml
index 438c24be9beb8..6b9f333b8222f 100644
--- a/test-suite/unit-tests/printing/proof_diffs_test.ml
+++ b/test-suite/unit-tests/printing/proof_diffs_test.ml
@@ -3,7 +3,9 @@ open Utest
 open Pp_diff
 open Proof_diffs
 
-let tokenize_string = !Pp_diff.tokenize_string
+let tokenize_string = Proof_diffs.tokenize_string
+let diff_pp = diff_pp ~tokenize_string
+let diff_str = diff_str ~tokenize_string
 
 let tests = ref []
 let add_test name test = tests := (mk_test name (TestCase test)) :: !tests