Ltrim trimming extra character - sql

Question

I have the below code:
SELECT
ltrim('REASON_ACTIVE_DCA', 'REASON_') reason
FROM
dual
However, I'm obtaining '_CTIVE_DCA'. What's happening and how can I get 'ACTIVE_DCA' with ltrim?

Answer 1

Because LTRIM() removes all the characters as a set. So all leading "R"s and "E"s and so on removed. In fact, the ordering of the characters in the second string is irrelevant, so you would get the same result with '_NOSAER'.
If you want to remove the leading string of REASON_ -- if present -- then you don't use trim(). Instead, one method is:
select (case when 'REASON_ACTIVE_DCA' LIKE 'REASON$_%' ESCAPE '$'
then substr('REASON_ACTIVE_DCA', 8)
else 'REASON_ACTIVE_DCA'
end)
There are other ways, such as:
select regexp_replace('REASON_ACTIVE_DCA', '^REASON_', '')

Answer 2

I would do it with regular string functions (not regular expressions), and using INSTR instead of LIKE so I don't have to worry about escaping underscore.
Something like this - including a few sample strings in the WITH clause for testing:
with
inputs (i_str) as (
select 'REASON_ACTIVE_DCA' from dual union all
select 'REASON_NOT_GIVEN' from dual union all
select null from dual union all
select 'REASON-SPECIAL' from dual union all
select 'REASON_' from dual union all
select 'REASON' from dual
)
select i_str, substr(i_str, case instr(i_str, 'REASON_')
when 1 then 1 + length('REASON_')
else 1 end) as new_str
from inputs;
I_STR NEW_STR
----------------- -----------------
REASON_ACTIVE_DCA ACTIVE_DCA
REASON_NOT_GIVEN NOT_GIVEN
REASON-SPECIAL REASON-SPECIAL
REASON_
REASON REASON