Ok, I've been testing your regex, mine is different because I wrote it according to [this guide](http://arxiv.org/help/arxiv_identifier), and it's for [this kind of expressions](http://refiddle.com/hi7); anyway, your's is correct for the arxiv API, because they don't allow versioning on the pre 2007 format (for example: [works](http://export.arxiv.org/api/query?search_query=all:physics/9901001), [doesn't work](http://export.arxiv.org/api/query?search_query=all:physics/9901001v1))