diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -2170,7 +2170,7 @@
         global _re2
         try:
             # check if match works, see issue3964
-            _re2 = bool(re2.match(r'\[([^\[]+)\]', b'[ui]'))
+            _re2 = bool(re2.match(br'\[([^\[]+)\]', b'[ui]'))
         except ImportError:
             _re2 = False