diff --git a/grawlix/sources/saxo.py b/grawlix/sources/saxo.py
index 77731ae..0d59b95 100644
--- a/grawlix/sources/saxo.py
+++ b/grawlix/sources/saxo.py
@@ -122,7 +122,7 @@ class Saxo(Source):
         :param url: Url of book
         :returns: Isbn of book
         """
-        isbn_match = re.search(f"\d+$", url)
+        isbn_match = re.search(r"\d+$", url)
         if isbn_match and isbn_match.group():
             return isbn_match.group()
         raise NotImplemented