string prefixes: don't normalise capital R-strings, normalise order

Reverse sort since that causes fewer changes on black's codebase.

Resolves psf#1244

black.py

@@ -2831,9 +2831,10 @@ def normalize_string_prefix(leaf: Leaf, remove_u_prefix: bool = False) -> None:
     match = re.match(r"^([furbFURB]*)(.*)$", leaf.value, re.DOTALL)
     assert match is not None, f"failed to match string {leaf.value!r}"
     orig_prefix = match.group(1)
-    new_prefix = orig_prefix.lower()
+    new_prefix = orig_prefix.replace("F", "f").replace("B", "b").replace("U", "u")
     if remove_u_prefix:
         new_prefix = new_prefix.replace("u", "")
+    new_prefix = "".join(sorted(new_prefix, reverse=True))
     leaf.value = f"{new_prefix}{match.group(2)}"
 
 

blib2to3/pgen2/tokenize.py

@@ -284,7 +284,7 @@ def compat(self, token: Tuple[int, Text], iterable: Iterable[TokenInfo]) -> None
 
 
 cookie_re = re.compile(r"^[ \t\f]*#.*?coding[:=][ \t]*([-\w.]+)", re.ASCII)
-blank_re = re.compile(br"^[ \t\f]*(?:[#\r\n]|$)", re.ASCII)
+blank_re = re.compile(rb"^[ \t\f]*(?:[#\r\n]|$)", re.ASCII)
 
 
 def _get_normal_name(orig_enc: str) -> str:

tests/data/string_prefixes.py

@@ -3,12 +3,16 @@
 name = R"Łukasz"
 F"hello {name}"
 B"hello"
+r"hello"
+fr"hello"
 
 # output
 
 
 #!/usr/bin/env python3.6
 
-name = r"Łukasz"
+name = R"Łukasz"
 f"hello {name}"
 b"hello"
+r"hello"
+rf"hello"