Re: correction suggestion for https://www.postgresql.org/docs/17/auth-username-maps.html

I wrote:
> "David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
>> I didn't add an example but felt the point "be referenced a single time
>> within" to be needed since, usefulness not withstanding, writing \1\1 for
>> database-username works but only the first instance of \1 is replaced.

> Hmm, I wonder if that isn't a bug we should fix. It's hard to believe
> anyone is relying on the second \1 *not* getting replaced, and perhaps
> there are use-cases for multiple replacements.

Here's a quick patch for that. I hacked up 003_peer.pl enough to
prove that multiple replacement works, but that test change is not
committable as-is because it assumes that the "system user" name
is "postgres". I don't like the existing test much either, since it
only tests the case of the substituted string being empty, which
means the substitution code could be quite broken and it wouldn't
notice. But I don't offhand see a way to improve that without
making assumptions about the incoming name...

regards, tom lane