diff --git a/addons/website_forum/models/forum.py b/addons/website_forum/models/forum.py
index c271b0cdef7..57f9313938f 100644
--- a/addons/website_forum/models/forum.py
+++ b/addons/website_forum/models/forum.py
@@ -386,6 +386,7 @@ class Post(models.Model):
         forum = self.env['forum.forum'].browse(forum_id)
         if content and self.env.user.karma < forum.karma_dofollow:
             for match in re.findall(r'<a\s.*href=".*?">', content):
+                match = re.escape(match)  # replace parenthesis or special char in regex
                 content = re.sub(match, match[:3] + 'rel="nofollow" ' + match[3:], content)
 
         if self.env.user.karma <= forum.karma_editor: