Normalize a paper reference to a DOI-based work key

The register identifies a "work" (issue codecheckers/register#150) by its DOI. DOIs are case-insensitive, so the key must be lowercased or two certificates citing the same DOI in different case would render as two work directories - the same class of bug fixed for venue names in codecheckers/register#192.

Usage

normalize_work_key(paper_reference)

Arguments

paper_reference

The `Paper reference` / `config_yml$paper$reference` value, expected to be a DOI URL (`https://doi.org/10...`) for a work page to exist at all; anything else (a PDF URL, a non-DOI landing page) yields `NA` - such a certificate simply has no work page, per #150.

Value

The lowercased bare DOI, or `NA` if `paper_reference` is not a DOI