Improve f-string expression detection regex so ...

CHANGES.md

@@ -7,6 +7,8 @@
 - Add support for formatting Jupyter Notebook files (#2357)
 - Move from `appdirs` dependency to `platformdirs` (#2375)
 - Present a more user-friendly error if .gitignore is invalid (#2414)
+- The failsafe for accidentally added backslashes in f-string expressions has been
+  hardened to handle more edge cases during quote normalization (#2437)
 
 ### Integrations
 

src/black/strings.py

@@ -190,9 +190,9 @@ def normalize_string_quotes(s: str) -> str:
     if "f" in prefix.casefold():
         matches = re.findall(
             r"""
-            (?:[^{]|^)\{  # start of the string or a non-{ followed by a single {
+            (?:(?<!\{)|^)\{  # start of the string or a non-{ followed by a single {
                 ([^{].*?)  # contents of the brackets except if begins with {{
-            \}(?:[^}]|$)  # A } followed by end of the string or a non-}
+            \}(?:(?!\})|$)  # A } followed by end of the string or a non-}
             """,
             new_body,
             re.VERBOSE,

tests/data/string_quotes.py

@@ -51,6 +51,11 @@
 '\'{z}\' {y * " "}'
 '{y * x} \'{z}\''
 
+# We must bail out if changing the quotes would introduce backslashes in f-string
+# expressions. xref: https://github.com/psf/black/issues/2348
+f"\"{b}\"{' ' * (long-len(b)+1)}: \"{sts}\",\n"
+f"\"{a}\"{'hello' * b}\"{c}\""
+
 # output
 
 """"""
@@ -100,3 +105,8 @@
 f"{y * x} '{z}'"
 "'{z}' {y * \" \"}"
 "{y * x} '{z}'"
+
+# We must bail out if changing the quotes would introduce backslashes in f-string
+# expressions. xref: https://github.com/psf/black/issues/2348
+f"\"{b}\"{' ' * (long-len(b)+1)}: \"{sts}\",\n"
+f"\"{a}\"{'hello' * b}\"{c}\""