Oracle 18c

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CREATE TABLE table_name ( Names ) AS

SELECT 'AT0000730007.GTX' FROM DUAL UNION ALL

SELECT 'CH0012032048.GTX' FROM DUAL UNION ALL

SELECT 'DE0005550602.GTX' FROM DUAL UNION ALL

SELECT 'DE0007236101.GTX' FROM DUAL UNION ALL

SELECT 'DE000A1EWWW0.GTX' FROM DUAL UNION ALL

SELECT 'DE000ENAG999.GTX' FROM DUAL UNION ALL

SELECT 'DE000TUAG000.GTX' FROM DUAL UNION ALL

SELECT 'FI0009000681.GTX' FROM DUAL UNION ALL

SELECT 'FR0000120172.GTX' FROM DUAL UNION ALL

SELECT 'FR0000125007.GTX' FROM DUAL UNION ALL

SELECT 'IT0003132476.GTX' FROM DUAL UNION ALL

SELECT 'NL0000235190.GTX' FROM DUAL UNION ALL

SELECT 'NL0011794037.GTX' FROM DUAL UNION ALL

SELECT 'SE0000148884.GTX' FROM DUAL;

14 rows affected

SELECT *

FROM table_name

WHERE REGEXP_LIKE( names, '^[A-Z]{2}[A-Z0-9]{10}\.GTX$' );