From 1b03cd029a10b75f85af6688bf1f76f8f4394382 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Fri, 2 Apr 2021 20:26:52 +0530
Subject: [PATCH] EPUB2 metadata: Read ISBNs in identifier elements without
 schemes if they are valid ISBNs and no properly identified isbns are present

---
 src/calibre/ebooks/metadata/__init__.py |  7 +++++--
 src/calibre/ebooks/metadata/opf2.py     | 10 ++++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/src/calibre/ebooks/metadata/__init__.py b/src/calibre/ebooks/metadata/__init__.py
index f7c68f0975..a5ed3489bf 100644
--- a/src/calibre/ebooks/metadata/__init__.py
+++ b/src/calibre/ebooks/metadata/__init__.py
@@ -399,10 +399,13 @@ def check_isbn13(isbn):
     return False
 
 
-def check_isbn(isbn):
+def check_isbn(isbn, simple_sanitize=False):
     if not isbn:
         return None
-    isbn = re.sub(r'[^0-9X]', '', isbn.upper())
+    if simple_sanitize:
+        isbn = isbn.upper().replace('-', '').strip().replace(' ', '')
+    else:
+        isbn = re.sub(r'[^0-9X]', '', isbn.upper())
     il = len(isbn)
     if il not in (10, 13):
         return None
diff --git a/src/calibre/ebooks/metadata/opf2.py b/src/calibre/ebooks/metadata/opf2.py
index 00c49fe318..43f5f17a5f 100644
--- a/src/calibre/ebooks/metadata/opf2.py
+++ b/src/calibre/ebooks/metadata/opf2.py
@@ -971,6 +971,7 @@ class OPF(object):  # {{{
 
     def get_identifiers(self):
         identifiers = {}
+        schemeless = []
         for x in self.XPath(
             'descendant::*[local-name() = "identifier" and text()]')(
                     self.metadata):
@@ -993,6 +994,15 @@ class OPF(object):  # {{{
                     val = check_isbn(val.split(':')[-1])
                     if val is not None:
                         identifiers['isbn'] = val
+                else:
+                    schemeless.append(val)
+
+        if schemeless and 'isbn' not in identifiers:
+            for val in schemeless:
+                if check_isbn(val, simple_sanitize=True) is not None:
+                    identifiers['isbn'] = check_isbn(val)
+                    break
+
         return identifiers
 
     def set_identifiers(self, identifiers):