de-duplicate the two regexes

src/blib2to3/pgen2/tokenize.py

@@ -136,8 +136,8 @@ def _combinations(*l: str) -> Set[str]:
 )
 
 # beginning of a single quoted f-string. must not end with `{{` or `\N{`
-SingleLbrace = r"(?:\\N{|\\[^{]|{{|[^'\\{])*(?<!\\N){(?!{)"
-DoubleLbrace = r'(?:\\N{|\\[^{]|{{|[^"\\{])*(?<!\\N){(?!{)'
+SingleLbrace = r"(?:\\N{|\\[^{]|{{|[^\n'\\{])*(?<!\\N)({)(?!{)"
+DoubleLbrace = r'(?:\\N{|\\[^{]|{{|[^\n"\\{])*(?<!\\N)({)(?!{)'
 
 # beginning of a triple quoted f-string. must not end with `{{` or `\N{`
 Single3Lbrace = r"(?:\\N{|\\[^{]|{{|'(?!'')|[^'{\\])*(?<!\\N){(?!{)"
@@ -175,8 +175,8 @@ def _combinations(*l: str) -> Set[str]:
 _string_middle_double = r'(?:[^\n"\\]|\\.)*'
 
 # FSTRING_MIDDLE and LBRACE, must not end with a `{{` or `\N{`
-_fstring_middle_single = r"(?:\\N{|\\[^{]|{{|[^\n'{\\])*(?<!\\N)({)(?!{)"
-_fstring_middle_double = r'(?:\\N{|\\[^{]|{{|[^\n"{\\])*(?<!\\N)({)(?!{)'
+_fstring_middle_single = SingleLbrace
+_fstring_middle_double = DoubleLbrace
 
 # First (or only) line of ' or " string.
 ContStr = group(