Default wildcard rule missing from the PSL algorithm implementation

[braedon]

The PSL formal algorithm includes the following step:

• If no rules match, the prevailing rule is "*".

This rule means that if an explicit public suffix can't be found for a domain in the PSL, the (actual, not effective) TLD is treated as a public suffix. This allows new TLDs (or custom internal TLDs) to be handled without requiring an explicit update to the PSL.

(It also means that the only time the PSL algorithm doesn't return a registrable domain is when the domain is itself a public suffix, which helps disambiguate that case.)

The PSL project has tests for this in the standard test suite:

// Unlisted TLD.
checkPublicSuffix('example', null);
checkPublicSuffix('example.example', 'example.example');
checkPublicSuffix('b.example.example', 'example.example');
checkPublicSuffix('a.b.example.example', 'example.example');

Here's the output for the test domains in tldextract 5.1.2:

>>> tldextract.extract('example').registered_domain                                                                                                                                                                                                                          
''
>>> tldextract.extract('example.example').registered_domain
''
>>> tldextract.extract('b.example.example').registered_domain
''
>>> tldextract.extract('a.b.example.example').registered_domain
''

The last three results are turning '', when they should return example.example.

[elliotwutingfeng]

Good catch. There are bunch of issues related to this at the public suffix list repository, such as https://github.com/publicsuffix/list/issues/694

Also relevant is https://wiki.mozilla.org/Public_Suffix_List/platform.sh_Problem#Further_Information (under the Further Information heading) which comments on the implications of enforcing/not enforcing the "*" rule.

@john-kurkowski