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SELECT city
FROM (
SELECT 'xx' AS city FROM DUAL UNION
SELECT 'ax' FROM DUAL UNION
SELECT 'xa' FROM DUAL UNION
SELECT 'aa' FROM DUAL UNION
SELECT 'x' FROM DUAL UNION
SELECT 'a' FROM DUAL
) station
WHERE NOT REGEXP_LIKE(city, '^[^aeiou]|[^aeiou]$')
| CITY |
|---|
| a |
| aa |