I also found some more non ISBN as well but most without the schema attribute. Since the attribute it optional it's possible to utilize pattern matching testing to identify the different types as they differ quite a lot:
Quote:
ISBN
<dc:identifier opf:scheme="isbn">9780000000001</dc:identifier>
JDCN
<dc:identifier opf:scheme="jdcn">915869090000000000DD</dc:identifier>
UUID (GUID)
<dc:identifier opf:scheme="uuid">50f9f8b1-8a81-4dd5-b104-0766188d7d2c</dc:identifier>
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