diff --git a/src/calibre/utils/mreplace.py b/src/calibre/utils/mreplace.py
index c4b70836cd..eb2059f0d3 100644
--- a/src/calibre/utils/mreplace.py
+++ b/src/calibre/utils/mreplace.py
@@ -1,4 +1,5 @@
 # multiple replace from dictionnary : http://code.activestate.com/recipes/81330/
+from __future__ import unicode_literals
 __license__   = 'GPL v3'
 __copyright__ = '2010, sengian <sengian1 @ gmail.com>'
 __docformat__ = 'restructuredtext en'
@@ -22,7 +23,10 @@ class MReplace(UserDict):
     def compile_regex(self):
         if len(self.data) > 0:
             keys = sorted(self.data, key=len, reverse=True)
-            tmp = "(%s)" % "|".join(map(re.escape, keys))
+            if isinstance(keys[0], bytes):
+                tmp = b"(%s)" % b"|".join(map(re.escape, keys))
+            else:
+                tmp = "(%s)" % "|".join(map(re.escape, keys))
             if self.re != tmp:
                 self.re = tmp
                 if self.case_sensitive: