Re: Regular expression for lower case to upper case.

On Sat, Dec 10, 2022 at 6:32 AM Peter J. Holzer <hjp-pgsql@hjp.at> wrote:

> On 2022-12-10 11:00:48 +0000, Eagna wrote:
> > > RegExp by itself cannot do this. You have to match all parts of the
> > > input into different capturing groups, then use lower() combined
> > > with format() to build a new string. Putting the capturing groups
> > > into an array is the most useful option.
> >
> > OK - I *_kind_* of see what you're saying.
> >
> > There's a small fiddle here (https://dbfiddle.uk/rhw1AdBY) if you'd
> > care to give an outline of the solution that you propose.
>
> For example like this:
>
> INSERT INTO test VALUES
> ('abc_def_ghi');
>
> Let's say I want to uppercase the part between the two underscores.
>
> First use regexp_replace to split the string into three parts: One
> before the match, the match and one after the match:
>
> SELECT
>   regexp_replace(x, '(.*_)(.*)(_.*)', '\1'),
>   regexp_replace(x, '(.*_)(.*)(_.*)', '\2'),
>   regexp_replace(x, '(.*_)(.*)(_.*)', '\3')
> FROM test;
>

A bit too inefficient for my taste.
I was describing the following:

with parts as materialized (
    select regexp_match(
        'abc_def_ghi',
        '^([^_]*_)([^_]*_)([^_]*)$') as part_array
)
select format(
    '%s%s%s',
    part_array[1],
    upper(part_array[2]),
    part_array[3])
from parts;

David J.