Fix resolved_username regex

Author: dhirschf

ldapauthenticator/ldapauthenticator.py

@@ -302,7 +302,7 @@ def getConnection(userdn, username, password):
         if resolved_username is None:
             return None
 
-        resolved_username = re.subn(r"[^\\],", r"\,", resolved_username)[0]
+        resolved_username = re.subn(r"([^\\]),", r"\1\,", resolved_username)[0]
 
         bind_dn_template = self.bind_dn_template
         if isinstance(bind_dn_template, str):