diff --git a/rag/nlp/rag_tokenizer.py b/rag/nlp/rag_tokenizer.py
index eee013e61..bfe4887a6 100644
--- a/rag/nlp/rag_tokenizer.py
+++ b/rag/nlp/rag_tokenizer.py
@@ -281,34 +281,49 @@ class RagTokenizer:
                 print("[FW]", tks, s)
                 print("[BW]", tks1, s1)
 
-            diff = [0 for _ in range(max(len(tks1), len(tks)))]
-            for i in range(min(len(tks1), len(tks))):
-                if tks[i] != tks1[i]:
-                    diff[i] = 1
+            i, j, _i, _j = 0, 0, 0, 0
+            same = 0
+            while i + same < len(tks1) and j + same < len(tks) and tks1[i + same] == tks[j + same]:
+                same += 1
+            if same > 0: res.append(" ".join(tks[j: j + same]))
+            _i = i + same
+            _j = j + same
+            j = _j + 1
+            i = _i + 1
 
-            if s1 > s:
-                tks = tks1
-
-            i = 0
-            while i < len(tks):
-                s = i
-                while s < len(tks) and diff[s] == 0:
-                    s += 1
-                if s == len(tks):
-                    res.append(" ".join(tks[i:]))
-                    break
-                if s > i:
-                    res.append(" ".join(tks[i:s]))
-
-                e = s
-                while e < len(tks) and e - s < 5 and diff[e] == 1:
-                    e += 1
+            while i < len(tks1) and j < len(tks):
+                tk1, tk = "".join(tks1[_i:i]), "".join(tks[_j:j])
+                if tk1 != tk:
+                    if len(tk1) > len(tk):
+                        j += 1
+                    else:
+                        i += 1
+                    continue
 
+                if tks1[i] != tks[j]:
+                    i += 1
+                    j += 1
+                    continue
+                # backward tokens from_i to i are different from forward tokens from _j to j.
                 tkslist = []
-                self.dfs_("".join(tks[s:e + 1]), 0, [], tkslist)
+                self.dfs_("".join(tks[_j:j]), 0, [], tkslist)
                 res.append(" ".join(self.sortTks_(tkslist)[0][0]))
 
-                i = e + 1
+                same = 1
+                while i + same < len(tks1) and j + same < len(tks) and tks1[i + same] == tks[j + same]:
+                    same += 1
+                res.append(" ".join(tks[j: j + same]))
+                _i = i + same
+                _j = j + same
+                j = _j + 1
+                i = _i + 1
+
+            if _i < len(tks1):
+                assert _j < len(tks)
+                assert "".join(tks1[_i:]) == "".join(tks[_j:])
+                tkslist = []
+                self.dfs_("".join(tks[_j:]), 0, [], tkslist)
+                res.append(" ".join(self.sortTks_(tkslist)[0][0]))
 
         res = " ".join(self.english_normalize_(res))
         if self.DEBUG: