Oracle 18c

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select regexp_substr('monkey_eats_mango_everyday_202002.txt', '(\d+)\.', 1, 1, NULL, 1) as parameter12a from dual;

PARAMETER12A

202002

select to_char(last_day(to_date(regexp_substr('monkey_eats_mango_everyday_202002.txt', '(\d+)\.', 1, 1, NULL, 1), 'yyyymm')), 'yyyymmdd') as parameter12a from dual;

PARAMETER12A

20200229

select regexp_substr('donkey_eats_pines_cones_201943_7771234_everyday_202002.txt', '(\d+)\.', 1, 1, NULL, 1) as parameter12a from dual;

PARAMETER12A

202002

select to_char(last_day(to_date(regexp_substr('donkey_eats_pines_cones_201943_7771234_everyday_202002.txt', '(\d+)\.', 1, 1, NULL, 1), 'yyyymm')), 'yyyymmdd') as parameter12a from dual;

PARAMETER12A

20200229