diff --git a/sphinx/util/smartypants.py b/sphinx/util/smartypants.py
index 47f8b59b21..43f8bc7248 100644
--- a/sphinx/util/smartypants.py
+++ b/sphinx/util/smartypants.py
@@ -155,7 +155,7 @@ def educateQuotes(text: str, language: str = 'en') -> str:
 
     # Special case for decade abbreviations (the '80s):
     if language.startswith('en'):  # TODO similar cases in other languages?
-        text = re.sub(r"""'(?=\d{2}s)""", apostrophe, text, re.UNICODE)
+        text = re.sub(r"""'(?=\d{2}s)""", apostrophe, text, flags=re.UNICODE)
 
     close_class = r"""[^\ \t\r\n\[\{\(\-]"""
     dec_dashes = r"""–|—"""