> It doesn't address that I'm trying to get information out of the diagnostic string.
> It also doesn't address the far more common case of Perl code producing error
> messages to emulate perl's own.
Since perl doesnt make any promises about error messages id say that
any code doing this is not exactly entitled to complain when things
break. The fact that any given version of perl documents a particular
error doesnt mean that we promise to provide that error for ever more
in that situation.
I think this is dangerous. For a long time perl didn't make a lot of guarantees -- until the documentation was updated to detail them. But perldiag is right there detailing the error messages, and people do parse for them, every single day, because they have no other choice right now.