diff --git a/src/Init/Data/String.lean b/src/Init/Data/String.lean
index 241ff1f743..a677bfa848 100644
--- a/src/Init/Data/String.lean
+++ b/src/Init/Data/String.lean
@@ -22,3 +22,5 @@ public import Init.Data.String.Substring
 public import Init.Data.String.TakeDrop
 public import Init.Data.String.Modify
 public import Init.Data.String.Termination
+public import Init.Data.String.ToSlice
+public import Init.Data.String.Search
diff --git a/src/Init/Data/String/Modify.lean b/src/Init/Data/String/Modify.lean
index 26dac98ad4..66eb8fe73f 100644
--- a/src/Init/Data/String/Modify.lean
+++ b/src/Init/Data/String/Modify.lean
@@ -208,32 +208,6 @@ Examples:
 @[inline] def map (f : Char → Char) (s : String) : String :=
   mapAux f s s.startValidPos
 
-/--
-In the string `s`, replaces all occurrences of `pattern` with `replacement`.
-
-Examples:
-* `"red green blue".replace "e" "" = "rd grn blu"`
-* `"red green blue".replace "ee" "E" = "red grEn blue"`
-* `"red green blue".replace "e" "E" = "rEd grEEn bluE"`
--/
-def replace (s pattern replacement : String) : String :=
-  if h : pattern.rawEndPos.1 = 0 then s
-  else
-    have hPatt := Nat.zero_lt_of_ne_zero h
-    let rec loop (acc : String) (accStop pos : String.Pos.Raw) :=
-      if h : pos.byteIdx + pattern.rawEndPos.byteIdx > s.rawEndPos.byteIdx then
-        acc ++ accStop.extract s s.rawEndPos
-      else
-        have := Nat.lt_of_lt_of_le (Nat.add_lt_add_left hPatt _) (Nat.ge_of_not_lt h)
-        if Pos.Raw.substrEq s pos pattern 0 pattern.rawEndPos.byteIdx then
-          have := Nat.sub_lt_sub_left this (Nat.add_lt_add_left hPatt _)
-          loop (acc ++ accStop.extract s pos ++ replacement) (pos + pattern) (pos + pattern)
-        else
-          have := Nat.sub_lt_sub_left this (Pos.Raw.lt_next s pos)
-          loop acc accStop (pos.next s)
-      termination_by s.rawEndPos.1 - pos.1
-    loop "" 0 0
-
 /--
 Replaces each character in `s` with the result of applying `Char.toUpper` to it.
 
diff --git a/src/Init/Data/String/Search.lean b/src/Init/Data/String/Search.lean
new file mode 100644
index 0000000000..eee377fa22
--- /dev/null
+++ b/src/Init/Data/String/Search.lean
@@ -0,0 +1,56 @@
+/-
+Copyright (c) 2025 Lean FRO, LLC. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Markus Himmel
+-/
+module
+
+prelude
+public import Init.Data.String.Slice
+
+set_option doc.verso true
+
+/-!
+# String operations based on slice operations
+
+This file contains {name}`String` operations that are implemented by passing to
+{name}`String.Slice`.
+-/
+
+public section
+
+namespace String
+
+section
+open String.Slice Pattern
+
+variable {ρ : Type} {σ : Slice → Type}
+variable [∀ s, Std.Iterators.Iterator (σ s) Id (SearchStep s)]
+variable [∀ s, Std.Iterators.Finite (σ s) Id]
+variable [∀ s, Std.Iterators.IteratorLoop (σ s) Id Id]
+
+
+/--
+Constructs a new string obtained by replacing all occurrences of {name}`pattern` with
+{name}`replacement` in {name}`s`.
+
+This function is generic over all currently supported patterns. The replacement may be a
+{name}`String` or a {name}`String.Slice`.
+
+Examples:
+* {lean}`"red green blue".replace 'e' "" = "rd grn blu"`
+* {lean}`"red green blue".replace (fun c => c == 'u' || c == 'e') "" = "rd grn bl"`
+* {lean}`"red green blue".replace "e" "" = "rd grn blu"`
+* {lean}`"red green blue".replace "ee" "E" = "red grEn blue"`
+* {lean}`"red green blue".replace "e" "E" = "rEd grEEn bluE"`
+* {lean}`"aaaaa".replace "aa" "b" = "bba"`
+* {lean}`"abc".replace "" "k" = "kakbkck"`
+-/
+@[inline]
+def replace [ToForwardSearcher ρ σ] [ToSlice α] (s : String) (pattern : ρ)
+    (replacement : α) : String :=
+  s.toSlice.replace pattern replacement
+
+end
+
+end String
diff --git a/src/Init/Data/String/Slice.lean b/src/Init/Data/String/Slice.lean
index f227f77c84..8fc9111d2f 100644
--- a/src/Init/Data/String/Slice.lean
+++ b/src/Init/Data/String/Slice.lean
@@ -9,6 +9,7 @@ prelude
 public import Init.Data.String.Pattern
 public import Init.Data.Ord.Basic
 public import Init.Data.Iterators.Combinators.FilterMap
+public import Init.Data.String.ToSlice
 
 set_option doc.verso true
 
@@ -36,6 +37,12 @@ public section
 
 namespace String.Slice
 
+instance : HAppend String String.Slice String where
+  -- This implementation performs an unnecessary copy which could be avoided by providing a custom
+  -- C++ implementation for this instance. Note: if `String` had no custom runtime representation
+  -- at all, then this would be very easy to get right from Lean using `ByteArray.copySlice`.
+  hAppend s t := s ++ t.copy
+
 open Pattern
 
 /--
@@ -349,6 +356,28 @@ Examples:
 def dropPrefix [ForwardPattern ρ] (s : Slice) (pat : ρ) : Slice :=
   dropPrefix? s pat |>.getD s
 
+/--
+Constructs a new string obtained by replacing all occurrences of {name}`pattern` with
+{name}`replacement` in {name}`s`.
+
+This function is generic over all currently supported patterns. The replacement may be a
+{name}`String` or a {name}`String.Slice`.
+
+Examples:
+* {lean}`"red green blue".toSlice.replace 'e' "" = "rd grn blu"`
+* {lean}`"red green blue".toSlice.replace (fun c => c == 'u' || c == 'e') "" = "rd grn bl"`
+* {lean}`"red green blue".toSlice.replace "e" "" = "rd grn blu"`
+* {lean}`"red green blue".toSlice.replace "ee" "E" = "red grEn blue"`
+* {lean}`"red green blue".toSlice.replace "e" "E" = "rEd grEEn bluE"`
+* {lean}`"aaaaa".toSlice.replace "aa" "b" = "bba"`
+* {lean}`"abc".toSlice.replace "" "k" = "kakbkck"`
+-/
+def replace [ToForwardSearcher ρ σ] [ToSlice α] (s : Slice) (pattern : ρ) (replacement : α) :
+    String :=
+  (ToForwardSearcher.toSearcher s pattern).fold (init := "") (fun
+    | sofar, .matched .. => sofar ++ ToSlice.toSlice replacement
+    | sofar, .rejected start stop => sofar ++ s.replaceStartEnd! start stop)
+
 /--
 Removes the specified number of characters (Unicode code points) from the start of the slice.
 
diff --git a/src/Init/Data/String/ToSlice.lean b/src/Init/Data/String/ToSlice.lean
new file mode 100644
index 0000000000..eb8e52db88
--- /dev/null
+++ b/src/Init/Data/String/ToSlice.lean
@@ -0,0 +1,37 @@
+/-
+Copyright (c) 2025 Lean FRO, LLC. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Markus Himmel
+-/
+module
+
+prelude
+public import Init.Data.String.Defs
+
+/-!
+# The `ToSlice` typeclass
+-/
+
+public section
+
+namespace String
+
+/--
+Typeclass for types that have a conversion function to `String.Slice`. This typeclass is used to
+make some `String`/`String.Slice` functions polymorphic. As such, for now it is not intended that
+there instances of this beyond `ToSlice String` and `ToSlice String.Slice`.
+
+To convert arbitrary data into a string representation, see `ToString` and `Repr`.
+-/
+class ToSlice (α : Type u) where
+  toSlice : α → String.Slice
+
+@[inline]
+instance : ToSlice String.Slice where
+  toSlice := id
+
+@[inline]
+instance : ToSlice String where
+  toSlice := toSlice
+
+end String
diff --git a/src/Lean/Compiler/FFI.lean b/src/Lean/Compiler/FFI.lean
index c56612b4ce..ab4662a922 100644
--- a/src/Lean/Compiler/FFI.lean
+++ b/src/Lean/Compiler/FFI.lean
@@ -7,7 +7,7 @@ module
 
 prelude
 public import Init.System.FilePath
-import Init.Data.String.Modify
+import Init.Data.String.Search
 
 public section
 
diff --git a/src/Lean/Data/Lsp/Utf16.lean b/src/Lean/Data/Lsp/Utf16.lean
index 8d04c5c62f..4cdadab483 100644
--- a/src/Lean/Data/Lsp/Utf16.lean
+++ b/src/Lean/Data/Lsp/Utf16.lean
@@ -7,7 +7,6 @@ Authors: Marc Huisinga, Wojciech Nawrocki
 module
 
 prelude
-public import Init.Data.String
 public import Lean.Data.Lsp.BasicAux
 public import Lean.DeclarationRange
 
diff --git a/src/Lean/DocString/Markdown.lean b/src/Lean/DocString/Markdown.lean
index 649b159549..1d32f92579 100644
--- a/src/Lean/DocString/Markdown.lean
+++ b/src/Lean/DocString/Markdown.lean
@@ -12,7 +12,7 @@ import Init.Data.Ord
 public import Lean.DocString.Types
 public import Init.Data.String.TakeDrop
 import Init.Data.String.Iterator
-public import Init.Data.String.Modify
+public import Init.Data.String.Search
 
 set_option linter.missingDocs true
 
diff --git a/tests/lean/run/string_replace.lean b/tests/lean/run/string_replace.lean
new file mode 100644
index 0000000000..9a90ac0b6e
--- /dev/null
+++ b/tests/lean/run/string_replace.lean
@@ -0,0 +1,21 @@
+module
+
+def isVowel (c : Char) : Bool :=
+  c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u'
+
+#guard "education".replace isVowel "☃!" = "☃!d☃!c☃!t☃!☃!n"
+#guard "red green blue".replace "e" "" = "rd grn blu"
+#guard "red green blue".replace 'e' "" = "rd grn blu"
+#guard "red green blue".replace "ee" "E" = "red grEn blue"
+#guard "red green blue".replace "e" "E" = "rEd grEEn bluE"
+#guard "abc".replace "" "k" = "kakbkck"
+#guard "ℕ".replace "" "z" = "zℕz"
+#guard "𝔸".replace "" "z" = "z𝔸z"
+#guard "v̂".replace "" "z" = "zvẑz"
+#guard "aaaaa".replace "aa" "b" = "bba"
+#guard "v̂fℚo".replace ("ℕfℚoℤ".toSlice.drop 1 |>.dropEnd 1) ("☃🔭🌌".toSlice.dropEnd 1 |>.drop 1) = "v̂🔭"
+#guard ("abcde".toSlice.drop 1).replace (· == 'c') "C" = "bCde"
+#guard (("ac  bc  cc  cd".toSlice.split "  ").map (·.replace 'c' "C") |>.toList) = ["aC", "bC", "CC", "Cd"]
+#guard "red green blue".replace (fun c => c == 'u' || c == 'e') "" = "rd grn bl"
+#guard "aab".replace "ab" "X" = "aX"
+#guard " ℚℚ\n ".replace "ℚ\n" "\n" = " ℚ\n "
diff --git a/tests/lean/run/string_slice.lean b/tests/lean/run/string_slice.lean
index c5a9414f84..45272340cd 100644
--- a/tests/lean/run/string_slice.lean
+++ b/tests/lean/run/string_slice.lean
@@ -23,12 +23,14 @@ Tests for `String.Slice` functions
 #guard ("coffee tea water".toSlice.split ' ').toList == ["coffee".toSlice, "tea".toSlice, "water".toSlice]
 #guard ("coffee tea water".toSlice.split " tea ").toList == ["coffee".toSlice, "water".toSlice]
 #guard ("baaab".toSlice.split "aa").toList == ["b".toSlice, "ab".toSlice]
+#guard ("aababaaba".toSlice.split "ab").toList == ["a".toSlice, "".toSlice, "a".toSlice, "a".toSlice]
 
 #guard ("coffee tea water".toSlice.splitInclusive Char.isWhitespace).toList == ["coffee ".toSlice, "tea ".toSlice, "water".toSlice]
 #guard ("coffee tea water".toSlice.splitInclusive ' ').toList == ["coffee ".toSlice, "tea ".toSlice, "water".toSlice]
 #guard ("coffee tea water".toSlice.splitInclusive " tea ").toList == ["coffee tea ".toSlice, "water".toSlice]
 #guard ("a".toSlice.splitInclusive (fun (_ : Char) => true)).toList == ["a".toSlice]
 #guard ("baaab".toSlice.splitInclusive "aa").toList == ["baa".toSlice, "ab".toSlice]
+#guard ("aababaaba".toSlice.splitInclusive "ab").toList == ["aab".toSlice, "ab".toSlice, "aab".toSlice, "a".toSlice]
 
 #guard "red green blue".toSlice.drop 4 == "green blue".toSlice
 #guard "red green blue".toSlice.drop 10 == "blue".toSlice