Converting function instr from Oracle to PostgreSQL (sql only) - postgresql

Question

I am working on converting something from Oracle to PostgreSQL. In the Oracle file there is a function:
instr(string,substring,starting point,nth location)
Example:
instr(text, '$', 1, 3)
In PostgreSQL this does not exist, so I looked up an equivalent function (4 parameter is important).
I found:
The function strpos(str, sub) in Postgres is equivalent of instr(str, sub) in Oracle. Tried options via split_part (it didn't work out).
I need the same result only with standard functions Postgres (not own function).
Maybe someone will offer options, even redundant in code.

Answer 1

This may be done in pure SQL using string_to_array.
with tab(val) as (
select 'qwe$rty$123$456$78'
union all
select 'qwe$rty$123$'
union all
select '123$456$'
union all
select '123$456'
)
select
val
/*Oracle's signature: instr(string , substring [, position [, occurrence ] ])*/
, case
when
array_length(
string_to_array(substr(val /*string*/, 1 /*position*/), '$' /*substring*/),
1
) <= 3 /*occurrence*/
then 0
else
length(array_to_string((
string_to_array(substr(val /*string*/, 1 /*position*/), '$' /*substring*/)
)[:3/*occurrence*/],
'$'/*substring*/)
) + 1
end as instr
from tab
val
instr
qwe$rty$123$456$78
12
qwe$rty$123$
12
123$456$
0
123$456
0
Postgres: fiddle
Oracle: fiddle