Assuming Calibre is configured to get metadata from the book:
Drag and drop the Book file (epub) onto the book list
Or
Select add Books: <point it to the book>
Or
drop the book into the Auto-Add folder (Use the Calibre Configuration: adding books: Auto... (tab) to specify)
IF you must use the Get from filename,
you need to have a matching (the file name convention being used) Template
Your example uses
Code:
(?P<title>.+) - (?P<author>[^_]+)