diff --git a/pycodestyle.py b/pycodestyle.py
index 4ade0871..00ec347f 100755
--- a/pycodestyle.py
+++ b/pycodestyle.py
@@ -133,7 +133,7 @@ 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(r'u?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*([=!]=)'