Fix bug which causes f-expressions to be split (#1809)

Closes #1807.

src/black/__init__.py

@@ -3611,13 +3611,14 @@ class StringSplitter(CustomSplitMapMixin, BaseStringSplitter):
     MIN_SUBSTR_SIZE = 6
     # Matches an "f-expression" (e.g. {var}) that might be found in an f-string.
     RE_FEXPR = r"""
-    (?<!\{)\{
+    (?<!\{) (?:\{\{)* \{ (?!\{)
         (?:
             [^\{\}]
             | \{\{
             | \}\}
+            | (?R)
         )+?
-    (?<!\})(?:\}\})*\}(?!\})
+    (?<!\}) \} (?:\}\})* (?!\})
     """
 
     def do_splitter_match(self, line: Line) -> TMatchResult:

tests/data/long_strings__regression.py

@@ -371,6 +371,10 @@ def xxxxxxx_xxxxxx(xxxx):
     ),
 }
 
+# We do NOT split on f-string expressions.
+print(f"Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam. {[f'{i}' for i in range(10)]}")
+x = f"This is a long string which contains an f-expr that should not split {{{[i for i in range(5)]}}}."
+
 # output
 
 
@@ -830,3 +834,13 @@ def xxxxxxx_xxxxxx(xxxx):
         r"(?<!([0-9]\ ))(?<=(^|\ ))([A-Z]+(\ )?|[0-9](\ )|[a-z](\\\ )){4,7}([A-Z]|[0-9]|[a-z])($|\b)(?!(\ ?([0-9]\ )|(\.)))"
     ),
 }
+
+# We do NOT split on f-string expressions.
+print(
+    "Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam."
+    f" {[f'{i}' for i in range(10)]}"
+)
+x = (
+    "This is a long string which contains an f-expr that should not split"
+    f" {{{[i for i in range(5)]}}}."
+)