how to extract last day date of the month from this string ‘opimus_rise_issue_command_201912.txt’

i want last date of the corresponding month and year from the string

'opimus_rise_issue_command_201912.txt'

expected output–

20191231

Answer

A few nested functions might help.

SQL> with test (col) as
  2    (select 'opimus_rise_issue_command_201912.txt' from dual)
  3  select to_char(last_day(to_date(regexp_substr(col, 'd+'), 'yyyymm')), 'yyyymmdd') result
  4  from test
  5  /

RESULT
--------------------------------------------------
20191231

SQL>