HV-920 Adding a regular expression based URL constraint validator which ...

...needs to be configured via XML or via a ConstraintDefinitionContributor
> It is a conceptual problem, just as it's a problem to expose internal types as method parameters of public APIs

I disagree, OSGi aside I would not have a problem with extending an internal class. That's totally different to have internal method parameters for public API. That I am extending an internal class is an implementation detail imo. 

> Consequently, it is an issue in OSGi 

OSGi is an issue

> which reminds me that the new public package should be added to the generated manifest

true.

Do we agree in skipping the abstract base class and adjusting the manifest to make this all work?