DOI, arXiv ID, PMID, ORCID, ISBN and ISSN: identifier formats explained

Last reviewed on October 3, 2026

A persistent identifier is a short string that resolves, today and ten years from now, to a particular work or person. In an academic PDF, identifiers are the most reliable thing an extractor can find. They are short, they have a known shape, and once you have one you can ask a metadata service for the canonical record. This page is a quick reference to the identifiers that show up in scholarly documents and how to recognise them.

DOI — Digital Object Identifier

A DOI looks like 10.1234/abcd.5678: a registrant prefix beginning with "10.", a slash, and a suffix chosen by the publisher. The prefix has at least four digits and may have more; the suffix may contain letters, digits, and a small set of punctuation. The CrossRef recommendation for matching DOIs in plain text is something close to 10.\d{4,9}/[-._;()/:A-Z0-9]+, case-insensitive.

DOIs are issued by registration agencies (CrossRef, DataCite, mEDRA, others) and resolve through doi.org. Once you have a DOI, you have everything: a canonical landing page, a structured metadata record from the registration agency's API, and an unambiguous reference for citation. A DOI in a citation is the single fastest way to clean that citation up.

Two pitfalls worth knowing. First, DOIs in PDF text often span a line break, with the hyphen used for the break sometimes belonging to the DOI itself; an extractor must stitch the lines together carefully. Second, a publisher's URL ("publisher.com/article/12345") is not a DOI — it can change at any time. Treat URLs and DOIs as different fields.

arXiv ID

arXiv assigns identifiers in two formats. New-style identifiers (since April 2007) look like 2401.12345 — four digits for the year and month, a dot, and a sequence number (four digits from 2007 to 2014, five digits since January 2015). They may carry a version suffix ("v2"). Old-style identifiers from before 2007 look like cs.CL/0401001: a category, a slash, the year/month, and a sequence number. Both forms appear inside PDFs, often on the first page as part of "arXiv:2401.12345 [cs.CL]".

An ArXiv ID resolves at arxiv.org/abs/<id> and the arXiv API returns a structured record. Many arXiv preprints are later published in a peer-reviewed venue and acquire a DOI; the arXiv record links to the DOI when one is registered. Since 2022 arXiv also registers its own DataCite DOI for every paper, of the form 10.48550/arXiv.2401.12345; that DOI points to the preprint, not to any later journal version. For a citation tool, the rule of thumb is: prefer the publisher's DOI when present, fall back to the arXiv ID (or arXiv DOI) when only the preprint exists. The arXiv identifier guide covers versions, categories and lookups in more detail.

PubMed ID and PubMed Central ID

PubMed IDs (PMIDs) are bare integers — typically eight digits in current use — referring to records in the PubMed bibliographic database. PubMed Central IDs (PMCIDs) take the form PMC1234567. In a PDF the PMID often appears next to a label, as in "PMID: 36720123", which makes them easy to extract with a simple pattern. Both identifiers resolve through the NCBI E-utilities API, which returns a structured record including the corresponding DOI when one exists.

ORCID

ORCID identifies researchers, not works. An ORCID iD looks like 0000-0002-1825-0097: four groups of four characters separated by hyphens, with the last character being either a digit or "X" (used as a check digit for the value 10). They are usually shown as URLs of the form https://orcid.org/0000-0002-1825-0097.

ORCID iDs are useful for disambiguating common author names. An extractor can pick them up with a straightforward pattern, but most PDFs include them only on the first page near the author block, sometimes encoded as a hyperlink rather than visible text.

ISBN and ISSN

ISBNs identify books, ISSNs identify serials. ISBN-13 has thirteen digits with hyphens in known positions ("978-0-12-345678-9"); ISBN-10 is the older, ten-character form. Both have a check digit at the end, which an extractor can validate to discard false matches.

ISSNs identify journal-level publications and look like 1234-567X. They are useful for grouping citations by venue but they do not by themselves point to a specific article.

Less common but worth recognising

Putting them together

A reasonable extraction pass is:

  1. Scan the first two pages and the references section for any of the patterns above.
  2. Validate where possible (ISBN check digits, DOI prefix shape, ORCID check character).
  3. Deduplicate: a single citation might give you a DOI and a PubMed ID; keep both, but prefer the DOI as the primary key.
  4. Stop. Do not try to derive a DOI from a publisher URL by parsing the URL — the relationship is not stable.

Once the identifiers are out, the heavy lifting moves to the metadata services that own them.

Related