How does wordpress name resized image copies?

yes, the naming convention for resized images is name-{width}x{height}.extension.

Is it good enough info to base a regex on? in practice most likely yes, but in theory no. You just can’t know if mark-20×20.jpg was not simply uploaded that way, and even in places where you know that a resized image should be used you have no way to know if wordpress had decided that the best fitting image is the original.
The proper way to know what is the original image based on image url is to search the attachments part of the DB for it which can be a little PITA (no simple API for it as far as I know).

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