Syntax described at http://www.doi.org/doi_handbook/2_Numbering.html#2.2 does not specify case. Uppercase recommended.
very long final portion should be handled as exceptions in associated Constraint:Format
The regular expression syntax described for "modern Crossref DOIs" by Andrew Gilmartin, a member of the U.S. Crossref team. Matches 74.4 million of the 74.9 million DOIs in Crossref.
Syntax described by Andrew Gilmartin (member of the U.S. Crossref team) for early DOIs (catches approximately 300,000 more DOIs than the "modern Crossref DOI" regular expression). Escape character added to avoid malformed input error.
DOI
serial code used to uniquely identify digital objects like academic papers (use upper case letters only)
digitaler Identifikator für physische, digitale oder abstrakte Objekte
Digital Object Identifier