Escape backslashes so set_key values round-trip

CHANGELOG.md

@@ -10,6 +10,7 @@ project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 ### Fixed
 
 - Strip a leading UTF-8 BOM from `.env` file contents so the first variable is no longer silently lost when the file is saved with BOM (e.g. by some JetBrains IDEs on Windows) by [@h1whelan] in [#640]
+- Escape backslashes in `set_key` so values containing backslashes (e.g. Windows paths, regexes) round-trip instead of being corrupted, and fix the single-quoted-value parser so a trailing backslash no longer breaks the file by [@eeshsaxena] in [#673]
 
 ## [1.2.2] - 2026-03-01
 
@@ -435,6 +436,7 @@ os.PathLike]` instead of just `os.PathLike` (#347 by [@bbc2]).
 [#497]: https://github.com/theskumar/python-dotenv/pull/497
 [#161]: https://github.com/theskumar/python-dotenv/issues/161
 [#640]: https://github.com/theskumar/python-dotenv/pull/640
+[#673]: https://github.com/theskumar/python-dotenv/pull/673
 [790c5c0]: https://github.com/theskumar/python-dotenv/commit/790c5c02991100aa1bf41ee5330aca75edc51311
 
 <!-- contributors -->
@@ -455,6 +457,7 @@ os.PathLike]` instead of just `os.PathLike` (#347 by [@bbc2]).
 [@eaf]: https://github.com/eaf
 [@earlbread]: https://github.com/earlbread
 [@eekstunt]: https://github.com/eekstunt
+[@eeshsaxena]: https://github.com/eeshsaxena
 [@eggplants]: https://github.com/eggplants
 [@ekohl]: https://github.com/ekohl
 [@elbehery95]: https://github.com/elbehery95

src/dotenv/main.py

@@ -216,7 +216,11 @@ def set_key(
     )
 
     if quote:
-        value_out = "'{}'".format(value_to_set.replace("'", "\\'"))
+        # The single-quoted-value parser treats backslash as an escape
+        # character, so backslashes must be escaped before single quotes
+        # (order matters) for the written value to round-trip unchanged.
+        escaped = value_to_set.replace("\\", "\\\\").replace("'", "\\'")
+        value_out = "'{}'".format(escaped)
     else:
         value_out = value_to_set
     if export:

src/dotenv/parser.py

@@ -22,7 +22,9 @@ def make_regex(string: str, extra_flags: int = 0) -> Pattern[str]:
 _single_quoted_key = make_regex(r"'([^']+)'")
 _unquoted_key = make_regex(r"([^=\#\s]+)")
 _equal_sign = make_regex(r"(=[^\S\r\n]*)")
-_single_quoted_value = make_regex(r"'((?:\\'|[^'])*)'")
+# Treat every ``\<char>`` as a single escaped unit (including ``\\``), so a
+# value ending in an escaped backslash does not merge with the closing quote.
+_single_quoted_value = make_regex(r"'((?:\\[\s\S]|[^'\\])*)'")
 _double_quoted_value = make_regex(r'"((?:\\"|[^"])*)"')
 _unquoted_value = make_regex(r"([^\r\n]*)")
 _comment = make_regex(r"(?:[^\S\r\n]*#[^\r\n]*)?")

tests/test_main.py

@@ -54,6 +54,22 @@ def test_set_key(dotenv_path, before, key, value, expected, after):
     mock_warning.assert_not_called()
 
 
+@pytest.mark.parametrize(
+    "value",
+    [
+        r"C:\Users",  # Windows path
+        r"\d+",  # regex
+        "a\\\\b",  # embedded double backslash
+        "C:\\",  # trailing backslash (previously corrupted the file)
+        "x\\'y",  # backslash before a quote
+    ],
+)
+def test_set_key_backslash_round_trip(dotenv_path, value):
+    dotenv.set_key(dotenv_path, "a", value)
+
+    assert dotenv.get_key(dotenv_path, "a") == value
+
+
 def test_set_key_encoding(dotenv_path):
     encoding = "latin-1"
 

tests/test_parser.py

@@ -273,6 +273,28 @@
                 )
             ],
         ),
+        (
+            "a='b\\\\c'",  # escaped backslash decodes to a single backslash
+            [
+                Binding(
+                    key="a",
+                    value="b\\c",
+                    original=Original(string="a='b\\\\c'", line=1),
+                    error=False,
+                )
+            ],
+        ),
+        (
+            "a='b\\\\'",  # trailing escaped backslash must not consume the quote
+            [
+                Binding(
+                    key="a",
+                    value="b\\",
+                    original=Original(string="a='b\\\\'", line=1),
+                    error=False,
+                )
+            ],
+        ),
         (
             'a="b\\"c"',
             [