From c11c497423aba9213089a5ea0a0a6de19f820508 Mon Sep 17 00:00:00 2001
From: by-ae <25704492+by-ae@users.noreply.github.com>
Date: Fri, 23 Oct 2026 10:16:53 +0000
Subject: [PATCH 1/2] Don't produce equivalent case mutants inside string
 escape sequences

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
---
 src/mutmut/mutation/mutators.py | 37 ++++++++++++++++++++++++++++-----
 tests/e2e/test_e2e_my_lib.py    |  1 -
 tests/mutation/test_mutation.py |  2 +-
 3 files changed, 33 insertions(+), 7 deletions(-)

diff --git a/src/mutmut/mutation/mutators.py b/src/mutmut/mutation/mutators.py
index d6fa5252..f4ff29c9 100644
--- a/src/mutmut/mutation/mutators.py
+++ b/src/mutmut/mutation/mutators.py
@@ -19,8 +19,17 @@
     ]
 ]
 
-# pattern to match (nearly) all chars in a string that are not part of an escape sequence
-NON_ESCAPE_SEQUENCE = re.compile(r"((?<!\\)[^\\]+)")
+# Pattern matching either: (1) escape sequences with case-sensitive parts (preserved as group None),
+# or (2) runs of non-backslash characters (captured as group 1 for case transformation).
+# For non-raw strings, preserve \xHH, \uHHHH, \UHHHHHHHH, \N{NAME} to avoid equivalent mutants.
+NON_ESCAPE_SEQUENCE = re.compile(
+    r"\\x[0-9a-fA-F]{2}"  # \xHH
+    r"|\\u[0-9a-fA-F]{4}"  # \uHHHH
+    r"|\\U[0-9a-fA-F]{8}"  # \UHHHHHHHH
+    r"|\\N\{[^}]+\}"  # \N{NAME}
+    r"|(\\.)"  # other escapes like \\, \n, \t (group 1, but treat as escape)
+    r"|((?<!\\)[^\\]+)"  # OR non-backslash run (group 2, to be transformed)
+)
 
 
 def operator_number(node: cst.BaseNumber) -> Iterable[cst.BaseNumber]:
@@ -53,13 +62,31 @@ def operator_string(node: cst.BaseString) -> Iterable[cst.BaseString]:
             # that mutation is meaningless for
             return
 
+        # For raw strings (r"..." or R"..."), escape sequences are literal, so case changes are NOT equivalent
+        is_raw = "r" in prefix.lower()
+
         supported_str_mutations: list[Callable[[str], str]] = [
             lambda x: "XX" + x + "XX",
-            # do not modify escape sequences, as this could break python syntax
-            lambda x: NON_ESCAPE_SEQUENCE.sub(lambda match: match.group(1).lower(), x),
-            lambda x: NON_ESCAPE_SEQUENCE.sub(lambda match: match.group(1).upper(), x),
         ]
 
+        if is_raw:
+            # Raw strings: use original regex (all non-backslash chars)
+            old_pattern = re.compile(r"((?<!\\)[^\\]+)")
+            supported_str_mutations.extend(
+                [
+                    lambda x: old_pattern.sub(lambda m: m.group(1).lower(), x),
+                    lambda x: old_pattern.sub(lambda m: m.group(1).upper(), x),
+                ]
+            )
+        else:
+            # Regular strings: preserve escape sequences with case-sensitive hex/names
+            supported_str_mutations.extend(
+                [
+                    lambda x: NON_ESCAPE_SEQUENCE.sub(lambda m: m.group(2).lower() if m.group(2) else m.group(0), x),
+                    lambda x: NON_ESCAPE_SEQUENCE.sub(lambda m: m.group(2).upper() if m.group(2) else m.group(0), x),
+                ]
+            )
+
         for mut_func in supported_str_mutations:
             new_value = f"{prefix}{value[0]}{mut_func(value[1:-1])}{value[-1]}"
             if new_value == value:
diff --git a/tests/e2e/test_e2e_my_lib.py b/tests/e2e/test_e2e_my_lib.py
index ec5263a2..7139e9a9 100644
--- a/tests/e2e/test_e2e_my_lib.py
+++ b/tests/e2e/test_e2e_my_lib.py
@@ -85,7 +85,6 @@ def test_my_lib_result_snapshot():
                 "my_lib.x_escape_sequences__mutmut_2": 0,
                 "my_lib.x_escape_sequences__mutmut_3": 1,
                 "my_lib.x_escape_sequences__mutmut_4": 0,
-                "my_lib.x_escape_sequences__mutmut_5": 0,
                 "my_lib.x_create_a_segfault_when_mutated__mutmut_1": -11,
                 "my_lib.x_create_a_segfault_when_mutated__mutmut_2": 0,
                 "my_lib.x_create_a_segfault_when_mutated__mutmut_3": 0,
diff --git a/tests/mutation/test_mutation.py b/tests/mutation/test_mutation.py
index 8ee2a644..283cbff2 100644
--- a/tests/mutation/test_mutation.py
+++ b/tests/mutation/test_mutation.py
@@ -234,7 +234,7 @@ def mutated_module(source: str) -> str:
             r'a: str = "Fo\N{ghost} \U11223344"',
             [
                 r'a: str = "XXFo\N{ghost} \U11223344XX"',
-                r'a: str = "FO\N{GHOST} \U11223344"',
+                r'a: str = "FO\N{ghost} \U11223344"',
                 r'a: str = "fo\N{ghost} \U11223344"',
                 "a: str = None",
             ],

From 8ca9f497c1eed19be4fc03272378fe7ec8d29842 Mon Sep 17 00:00:00 2001
From: by-ae <25704492+by-ae@users.noreply.github.com>
Date: Fri, 23 Oct 2026 10:17:21 +0000
Subject: [PATCH 2/2] Add tests for case mutations around escape sequences

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
---
 tests/mutation/test_mutation.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/tests/mutation/test_mutation.py b/tests/mutation/test_mutation.py
index 283cbff2..d5ca2ea9 100644
--- a/tests/mutation/test_mutation.py
+++ b/tests/mutation/test_mutation.py
@@ -239,6 +239,14 @@ def mutated_module(source: str) -> str:
                 "a: str = None",
             ],
         ),
+        # Escape sequences with hex digits should not produce equivalent case-flip mutants
+        (r'"\u00a0"', [r'"XX\u00a0XX"']),
+        (r'"\U0001f600"', [r'"XX\U0001f600XX"']),
+        (r'"\x7f"', [r'"XX\x7fXX"']),
+        (r'"\N{GREEK CAPITAL LETTER DELTA}"', [r'"XX\N{GREEK CAPITAL LETTER DELTA}XX"']),
+        (r'"Hello\u00a0World"', [r'"XXHello\u00a0WorldXX"', r'"hello\u00a0world"', r'"HELLO\u00a0WORLD"']),
+        # Raw strings: escapes are literal, so case changes are NOT equivalent
+        (r'r"\u00a0"', [r'r"XX\u00a0XX"', r'r"\u00A0"']),
         ("lambda: 0", ["lambda: 1", "lambda: None"]),
         ("1 in (1, 2)", ["2 in (1, 2)", "1 not in (1, 2)", "1 in (2, 2)", "1 in (1, 3)"]),
         ("1+1", ["2+1", "1 - 1", "1+2"]),