Fix regular expression that black uses to identify f-expressions (#2287)

Fixes #1469

CHANGES.md

@@ -12,6 +12,7 @@
 - Add extra uvloop install + import support if in python env (#2258)
 - Fix --experimental-string-processing crash when matching parens are not found (#2283)
 - Make sure to split lines that start with a string operator (#2286)
+- Fix regular expression that black uses to identify f-expressions (#2287)
 
 ### _Blackd_
 

src/black/trans.py

@@ -971,8 +971,8 @@ class StringSplitter(CustomSplitMapMixin, BaseStringSplitter):
             | \{\{
             | \}\}
             | (?R)
-        )+?
-    (?<!\}) \} (?:\}\})* (?!\})
+        )+
+    \}
     """
 
     def do_splitter_match(self, line: Line) -> TMatchResult:

tests/data/long_strings__regression.py

@@ -407,6 +407,12 @@ def _legacy_listen_examples():
     )
 
 
+temp_msg = (
+    f"{f'{humanize_number(pos)}.': <{pound_len+2}} "
+    f"{balance: <{bal_len + 5}} "
+    f"<<{author.display_name}>>\n"
+)
+
 assert str(suffix_arr) == (
     "['$', 'angaroo$', 'angrykangaroo$', 'aroo$', 'garoo$', "
     "'grykangaroo$', 'kangaroo$', 'ngaroo$', 'ngrykangaroo$', "
@@ -942,6 +948,12 @@ def _legacy_listen_examples():
     )
 
 
+temp_msg = (
+    f"{f'{humanize_number(pos)}.': <{pound_len+2}} "
+    f"{balance: <{bal_len + 5}} "
+    f"<<{author.display_name}>>\n"
+)
+
 assert (
     str(suffix_arr)
     == "['$', 'angaroo$', 'angrykangaroo$', 'aroo$', 'garoo$', "