diff --git a/LICENSE b/LICENSE
deleted file mode 100644
index 165238e..0000000
--- a/LICENSE
+++ /dev/null
@@ -1,21 +0,0 @@
-MIT License
-
-Copyright (c) 2026 Sven van Heugten
-
-Permission is hereby granted, free of charge, to any person obtaining a copy
-of this software and associated documentation files (the "Software"), to deal
-in the Software without restriction, including without limitation the rights
-to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-copies of the Software, and to permit persons to whom the Software is
-furnished to do so, subject to the following conditions:
-
-The above copyright notice and this permission notice shall be included in all
-copies or substantial portions of the Software.
-
-THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-SOFTWARE.
diff --git a/README.md b/README.md
deleted file mode 100644
index 012eeb9..0000000
--- a/README.md
+++ /dev/null
@@ -1,9 +0,0 @@
-# mechanicaldiff
-
-`mechanicaldiff` lets you filter down a diff to _only_ the parts that make a specific mechanical change, so you can commit those changes separately.
-
-Usage: `git diff | mechanicaldiff <pattern> <replacement> | recountdiff`
-
-It differs from [grepdiff](https://linux.die.net/man/1/grepdiff) in that it doesn’t just search for the lines that contain the given `pattern`, but also checks that the _only_ change in that line is that `pattern` was replaced with the given `replacement`.
-
-It differs from simply replacing `pattern` with `replacement` in the entire repository in that it filters down an _existing_ diff, which means that the resulting diff doesn’t make the mechanical change everywhere, but only in the lines that your original diff also made it.
diff --git a/bin/mechanicaldiff b/bin/mechanicaldiff.py
similarity index 78%
rename from bin/mechanicaldiff
rename to bin/mechanicaldiff.py
index 8ac824d..11fc6c5 100755
--- a/bin/mechanicaldiff
+++ b/bin/mechanicaldiff.py
@@ -4,19 +4,9 @@ import re
 import sys
 
 
-def trim_blank_ends(lines: list[str]) -> list[str]:
-    start = 0
-    end = len(lines)
-    while start < end and lines[start] == "\n":
-        start += 1
-    while end > start and lines[end - 1] == "\n":
-        end -= 1
-    return lines[start:end]
-
-
 def filter_change_block(
     change_lines: list[str], search: str, replace: str
-) -> list[str]:
+) -> list[str] | None:
     removed_lines = []
     added_lines = []
     phase = "minus"
@@ -37,16 +27,14 @@ def filter_change_block(
     transformed_removed = [
         re.sub(search, replace, line) for line in removed_lines
     ]
-    if trim_blank_ends(transformed_removed) == trim_blank_ends(added_lines):
+    if transformed_removed == added_lines:
         return change_lines
 
+    if len(removed_lines) != len(added_lines):
+        return None
+
     kept_lines = []
-    max_len = max(len(removed_lines), len(added_lines))
-    for idx in range(max_len):
-        removed = removed_lines[idx] if idx < len(removed_lines) else None
-        added = added_lines[idx] if idx < len(added_lines) else None
-        if removed is None:
-            break
+    for removed, added in zip(removed_lines, added_lines):
         if re.sub(search, replace, removed) == added:
             kept_lines.append(f"-{removed}")
             kept_lines.append(f"+{added}")
@@ -54,16 +42,16 @@ def filter_change_block(
             kept_lines.append(f" {removed}")
 
     if not kept_lines:
-        return []
+        return None
 
     return kept_lines
 
 
 def filter_hunk(
     hunk_lines: list[str], search: str, replace: str
-) -> list[str]:
+) -> list[str] | None:
     if not hunk_lines:
-        return []
+        return None, False
 
     header = hunk_lines[0]
     body = hunk_lines[1:]
@@ -85,7 +73,7 @@ def filter_hunk(
         i += 1
 
     if not any(line.startswith(("+", "-")) for line in output_body):
-        return []
+        return None
 
     return [header] + output_body
 
@@ -97,8 +85,7 @@ def main() -> None:
     search = sys.argv[1]
     replace = sys.argv[2]
 
-    input_data = sys.stdin.read()
-    lines = input_data.splitlines(keepends=True)
+    lines = sys.stdin.read().splitlines(keepends=True)
     preamble_lines = []
     sections = []
     current = None
@@ -127,23 +114,29 @@ def main() -> None:
 
     output_lines = []
     output_lines.extend(preamble_lines)
+    left_out = False
+
     for section in sections:
         kept_hunks = []
         for hunk_lines in section["hunks"]:
             filtered = filter_hunk(hunk_lines, search, replace)
             if filtered:
+                if filtered != hunk_lines:
+                    left_out = True
                 kept_hunks.append(filtered)
+            elif hunk_lines:
+                left_out = True
 
         if not kept_hunks:
+            left_out = True
             continue
 
         output_lines.extend(section["header"])
         for hunk_lines in kept_hunks:
             output_lines.extend(hunk_lines)
 
-    output_data = "".join(output_lines)
-    sys.stdout.write(output_data)
-    if output_data != input_data:
+    sys.stdout.write("".join(output_lines))
+    if left_out:
         raise SystemExit(1)
 
 
diff --git a/default.nix b/default.nix
index 54cf645..79ed058 100644
--- a/default.nix
+++ b/default.nix
@@ -7,7 +7,6 @@
   python3Packages,
   shellcheck-minimal,
   shfmt,
-  patchutils_0_4_2,
 }:
 
 let
@@ -43,12 +42,11 @@ stdenv.mkDerivation {
     python3Packages.flake8
     shellcheck-minimal
     shfmt
-    patchutils_0_4_2
   ];
 
   checkPhase = ''
     runHook preCheck
-    flake8 bin/mechanicaldiff
+    flake8 bin/mechanicaldiff.py
     shellcheck test/mechanicaldiff.bats
     shfmt -d test/mechanicaldiff.bats
     bats test
@@ -57,7 +55,7 @@ stdenv.mkDerivation {
 
   installPhase = ''
     mkdir -p $out/bin
-    cp $src/bin/mechanicaldiff $out/bin/mechanicaldiff
+    cp $src/bin/mechanicaldiff.py $out/bin/mechanicaldiff
     chmod +x $out/bin/mechanicaldiff
   '';
 }
diff --git a/flake.nix b/flake.nix
index 387b65c..0593261 100644
--- a/flake.nix
+++ b/flake.nix
@@ -26,7 +26,6 @@
             pkgs.python3Packages.flake8
             pkgs.shellcheck
             pkgs.shfmt
-            pkgs.patchutils_0_4_2
           ];
         };
       }
diff --git a/test/mechanicaldiff.bats b/test/mechanicaldiff.bats
index 82c7ea1..beb4853 100755
--- a/test/mechanicaldiff.bats
+++ b/test/mechanicaldiff.bats
@@ -15,7 +15,7 @@ setup() {
   printf "%s\n" "bar" >"$BATS_TEST_TMPDIR/new"
   diff_output="$(git diff --no-index "$BATS_TEST_TMPDIR/old" \
     "$BATS_TEST_TMPDIR/new" || true)"
-  run mechanicaldiff foo bar <<<"$diff_output"
+  run mechanicaldiff.py foo bar <<<"$diff_output"
   assert_success
   assert_output "$diff_output"
 }
@@ -25,7 +25,7 @@ setup() {
   printf "%s\n" "baz" >"$BATS_TEST_TMPDIR/new"
   git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
     >"$BATS_TEST_TMPDIR/diff" || true
-  run mechanicaldiff foo bar <"$BATS_TEST_TMPDIR/diff"
+  run mechanicaldiff.py foo bar <"$BATS_TEST_TMPDIR/diff"
   assert_failure
   assert_output ""
 }
@@ -39,7 +39,7 @@ setup() {
   diff_output="$(git diff --no-index "$BATS_TEST_TMPDIR/old" \
     "$BATS_TEST_TMPDIR/new" || true)"
 
-  run mechanicaldiff foo bar <<<"$diff_output"
+  run mechanicaldiff.py foo bar <<<"$diff_output"
   assert_failure
   assert_output ""
 }
@@ -75,7 +75,7 @@ setup() {
     "$BATS_TEST_TMPDIR/new_kept" \
     >"$BATS_TEST_TMPDIR/diff_expected" || true
 
-  run mechanicaldiff "beta" "beta_changed" <"$BATS_TEST_TMPDIR/diff_full"
+  run mechanicaldiff.py "beta" "beta_changed" <"$BATS_TEST_TMPDIR/diff_full"
   assert_failure
   assert_output --partial "-beta"
   assert_output --partial "+beta_changed"
@@ -96,15 +96,15 @@ setup() {
   cat "$BATS_TEST_TMPDIR/diff_one" "$BATS_TEST_TMPDIR/diff_two" \
     >"$BATS_TEST_TMPDIR/diff_all"
 
-  run mechanicaldiff foo bar <"$BATS_TEST_TMPDIR/diff_one"
+  run mechanicaldiff.py foo bar <"$BATS_TEST_TMPDIR/diff_one"
   assert_success
   output_one="$output"
 
-  run mechanicaldiff foo bar <"$BATS_TEST_TMPDIR/diff_two"
+  run mechanicaldiff.py foo bar <"$BATS_TEST_TMPDIR/diff_two"
   assert_failure
   output_two="$output"
 
-  run mechanicaldiff foo bar <"$BATS_TEST_TMPDIR/diff_all"
+  run mechanicaldiff.py foo bar <"$BATS_TEST_TMPDIR/diff_all"
   assert_failure
   assert_output "${output_one}${output_two}"
 }
@@ -122,7 +122,7 @@ setup() {
   git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
     >"$BATS_TEST_TMPDIR/diff" || true
 
-  run mechanicaldiff 'foo\((\d+)\)' 'foo-\1' <"$BATS_TEST_TMPDIR/diff"
+  run mechanicaldiff.py 'foo\((\d+)\)' 'foo-\1' <"$BATS_TEST_TMPDIR/diff"
   assert_success
   assert_output "$(cat "$BATS_TEST_TMPDIR/diff")"
 }
@@ -138,109 +138,7 @@ setup() {
   git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
     >"$BATS_TEST_TMPDIR/diff" || true
 
-  run mechanicaldiff foo bar <"$BATS_TEST_TMPDIR/diff"
-  assert_success
-  assert_output "$(cat "$BATS_TEST_TMPDIR/diff")"
-}
-
-@test "matches when replacement removes entire line" {
-  cat >"$BATS_TEST_TMPDIR/old" <<-'EOF'
-	alpha
-	foo
-	omega
-	EOF
-  cat >"$BATS_TEST_TMPDIR/new" <<-'EOF'
-	alpha
-	omega
-	EOF
-
-  git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
-    >"$BATS_TEST_TMPDIR/diff" || true
-
-  run mechanicaldiff foo "" <"$BATS_TEST_TMPDIR/diff"
-  assert_success
-  assert_output "$(cat "$BATS_TEST_TMPDIR/diff")"
-}
-
-@test "keeps matching change with an extra removed line" {
-  cat >"$BATS_TEST_TMPDIR/old" <<-'EOF'
-	foo
-	bar
-	baz
-	EOF
-  cat >"$BATS_TEST_TMPDIR/new" <<-'EOF'
-	foo_changed
-	baz
-	EOF
-
-  git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
-    >"$BATS_TEST_TMPDIR/diff" || true
-
-  cat >"$BATS_TEST_TMPDIR/new_kept" <<-'EOF'
-	foo_changed
-	bar
-	baz
-	EOF
-  git diff --no-index "$BATS_TEST_TMPDIR/old" \
-    "$BATS_TEST_TMPDIR/new_kept" \
-    >"$BATS_TEST_TMPDIR/diff_expected" || true
-
-  run mechanicaldiff foo foo_changed <"$BATS_TEST_TMPDIR/diff"
-  assert_failure
-  expected="$(sed \
-    -e 's/new_kept/new/g' \
-    -e '/^index /d' \
-    "$BATS_TEST_TMPDIR/diff_expected")"
-  output_normalized="$(printf '%s\n' "$output" | recountdiff | sed -e '/^index /d')"
-  assert_equal "$output_normalized" "$expected"
-}
-
-@test "keeps matching change with an extra added line" {
-  cat >"$BATS_TEST_TMPDIR/old" <<-'EOF'
-	foo
-	baz
-	EOF
-  cat >"$BATS_TEST_TMPDIR/new" <<-'EOF'
-	foo_changed
-	bar
-	baz
-	EOF
-
-  git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
-    >"$BATS_TEST_TMPDIR/diff" || true
-
-  cat >"$BATS_TEST_TMPDIR/new_kept" <<-'EOF'
-	foo_changed
-	baz
-	EOF
-  git diff --no-index "$BATS_TEST_TMPDIR/old" \
-    "$BATS_TEST_TMPDIR/new_kept" \
-    >"$BATS_TEST_TMPDIR/diff_expected" || true
-
-  run mechanicaldiff foo foo_changed <"$BATS_TEST_TMPDIR/diff"
-  assert_failure
-  expected="$(sed \
-    -e 's/new_kept/new/g' \
-    -e '/^index /d' \
-    "$BATS_TEST_TMPDIR/diff_expected")"
-  output_normalized="$(printf '%s\n' "$output" | recountdiff | sed -e '/^index /d')"
-  assert_equal "$output_normalized" "$expected"
-}
-
-@test "ignores leading and trailing blank lines for comparison" {
-  cat >"$BATS_TEST_TMPDIR/old" <<-'EOF'
-
-	prefix foo suffix
-
-	EOF
-  cat >"$BATS_TEST_TMPDIR/new" <<-'EOF'
-	prefix bar suffix
-	EOF
-
-  git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
-    >"$BATS_TEST_TMPDIR/diff" || true
-
-  run mechanicaldiff foo bar <"$BATS_TEST_TMPDIR/diff"
+  run mechanicaldiff.py foo bar <"$BATS_TEST_TMPDIR/diff"
   assert_success
   assert_output "$(cat "$BATS_TEST_TMPDIR/diff")"
 }
@@ -263,14 +161,14 @@ setup() {
   git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new_kept" \
     >"$BATS_TEST_TMPDIR/diff_expected" || true
 
-  run mechanicaldiff foo foo_changed <"$BATS_TEST_TMPDIR/diff_full"
+  run mechanicaldiff.py foo foo_changed <"$BATS_TEST_TMPDIR/diff_full"
   assert_failure
   expected="$(sed \
     -e 's/new_kept/new/g' \
     -e '/^index /d' \
     "$BATS_TEST_TMPDIR/diff_expected")"
   output_normalized="$(printf '%s\n' "$output" | sed -e '/^index /d')"
-  assert_equal "$output_normalized" "$expected"
+  assert_equal "$expected" "$output_normalized"
 }
 
 @test "drops hunk when all pairs become context" {
@@ -287,7 +185,7 @@ setup() {
   git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
     >"$BATS_TEST_TMPDIR/diff_full" || true
 
-  run mechanicaldiff qux quux <"$BATS_TEST_TMPDIR/diff_full"
+  run mechanicaldiff.py qux quux <"$BATS_TEST_TMPDIR/diff_full"
   assert_failure
   assert_output ""
 }