There is no designated text prefix for the entire DOI system, though individual registration agencies offer test prefixes. DataCite previously offered a single test prefix 10.5072, however it was slightly different from example.org and 192.0.2.0/24 in that users could actually create test DOIs under that
prefix. This was problematic for a number of reasons, including the fact that some repository software had the default prefix set to 10.5072 and resulted in
a lot of DOIs mistakenly created under that prefix instead of
a real prefix. We retired 10.5072 in 2019 and launched an entire test instance of DataCite Fabrica, which is independent of the production system and allows users to get their own text prefixes.
In the DataCite context, for testing purposes, the test system should be used (see our Testing guide). For example purposes (when you need to show a DOI, but don’t necessarily need to create DOIs), you have some choices:
A big caveat with the DataCite test system is that DOIs created there are not registered in the global handle system, so they are not resolvable at https://doi.org/XXXXX/XXXX . The DataCite test system uses a test handle system at https://handle.stage.datacite.org , so test system “DOIs” look something like DataCite Schema .
This info is DataCite-specific; others like @pfeeney and @Jonathan_DOI may have additional input.