Re: add function argument names to regex* functions.

> > Another possibility is `index`, which is relatively short and not a
> > reserved keyword ^1. `position` is not as precise but would avoid the
> > conceptual overloading of ordinary indices.
>
> I'm not a fan of "index" since that leaves the question of
> whether it's 0 or 1 based. "Position" is a bit better, but I think
> Jian's suggestion of "occurance" is best.

We do have precedent for one-based `index` in Postgres: array types are
1-indexed by default! "Occurrence" removes that ambiguity but it's long
and easy to misspell (I looked it up after typing it just now and it
_still_ feels off).

How's "instance"?