Docstring may also be prefixed with f, U, R, and F.

pycodestyle.py

@@ -133,7 +133,9 @@ def lru_cache(maxsize=128):  # noqa as it's a fake implementation.
 RAISE_COMMA_REGEX = re.compile(r'raise\s+\w+\s*,')
 RERAISE_COMMA_REGEX = re.compile(r'raise\s+\w+\s*,.*,\s*\w+\s*$')
 ERRORCODE_REGEX = re.compile(r'\b[A-Z]\d{3}\b')
-DOCSTRING_REGEX = re.compile(r'u?r?["\']')
+DOCSTRING_REGEX = re.compile('(u|r|ur|ru' +
+                             ('|fr|rf' if sys.version_info >= (3, 6) else '') +
+                             r')?["\']', re.IGNORECASE)
 EXTRANEOUS_WHITESPACE_REGEX = re.compile(r'[\[({] | [\]}),;:]')
 WHITESPACE_AFTER_COMMA_REGEX = re.compile(r'[,;:]\s*(?:  |\t)')
 COMPARE_SINGLETON_REGEX = re.compile(r'(\bNone|\bFalse|\bTrue)?\s*([=!]=)'