Re: BUG #19602: Vuln46: citext split_part silently returns NULL for a zero field position instead of raising core sp

Hi Manu,

> Tested on master:
>
> - the citext regression tests pass, with new cases for all of the above;
> - ALTER EXTENSION citext UPDATE TO '1.9' changes the -1 case from NULL
> to 'c' in an existing database;
> - on 20000 random lower-case strings, delimiters of up to three
> characters (some of them regex metacharacters) and positions from -6
> to 6, the result matches the core split_part() in every case.

I think v1 can be simplified like this:

```
RETURN pg_catalog.split_part(
pg_catalog.regexp_replace(
$1::pg_catalog.text,
pg_catalog.regexp_replace($2::pg_catalog.text,
'([^a-zA-Z_0-9])', E'\\\\\\1', 'g'),
pg_catalog.replace($2::pg_catalog.text, E'\\', E'\\\\'),
'gi'),
$2::pg_catalog.text,
$3);
```

Without WHEN .. ELSE and COALESCE it should be faster too.

Thoughts?

--
Best regards,
Aleksander Alekseev