diff --git a/black.py b/black.py
index 3897ebae100..c3778acf4dc 100644
--- a/black.py
+++ b/black.py
@@ -2831,7 +2831,7 @@ 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", "")
     leaf.value = f"{new_prefix}{match.group(2)}"
diff --git a/tests/data/string_prefixes.py b/tests/data/string_prefixes.py
index fbad5e0704f..0ca3686a2b6 100644
--- a/tests/data/string_prefixes.py
+++ b/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"
+fR"hello"