Mismatch in PropertyIdValueImpl

[fer-rum]

There is a mismatch in PropertyIdValueImpl between the exception message and the reason why the exception was thrown:

        if (!id.matches("^P[1-9][0-9]*$")) {
            throw new IllegalArgumentException(
                    "Wikibase item ids must have the form \"Q[1-9]+\"");
        }

Is it intentional that no property with the id "P0" is allowed?

[julianmendez]

Also, the form "Q[1-9]+" does not capture Q20: "Norway".

[mkroetzsch]

It is intended that "0" is not allowed (that's just how things are in Wikibase; no deeper reason I guess). The exceptions text is of course wrong. Will fix.

[fer-rum]

Can you please add a documentation statement about the appropriate values (or add a link to the documentation), since there yet seems to be none (at least I didn't find some).
Also please note that some people consider positive integers to include 0.

[mkroetzsch]

I extended the documentation in 37be7b7. I think it is generally agreed upon what "positive integer" means. At least I found the same definition in all places I looked. If 0 is included, one usually speaks of non-negative integers.

[fer-rum]

I think that will do it.