Custom identifier: Is it possible to

I have a custom identifier with https://openlibrary.org/books/{id} as a rule.

I'm having a bit of trouble, though.

If I paste https://openlibrary.org/books/OL8604930M/A_Baker's_Field_Guide_to_Chocolate_Chip_Cookies in, I get:

OL8604930M/A_Baker's_Field_Guide_to_Chocolate_Chip_Cookies

I don't want that second part, just the OL bit. Adding a trailing slash to the rule works, but then if the URL ends with /books/OL8604930M it fails to detect and I get a url.

Aside from making two rules (which results in duplicate links), is there a way to set the trailing slash as optional?