MariaDB 10.2

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SELECT VERSION();

VERSION()

10.2.27-MariaDB-10.2.27+maria~jessie-log

SELECT REGEXP_REPLACE('567890', '[7]', 'z');

REGEXP_REPLACE('567890', '[7]', 'z')

56z890

DROP TABLE IF EXISTS `test`;
CREATE TABLE IF NOT EXISTS `test` (
  `field` TEXT
);
INSERT INTO `test` (`field`)
VALUES ('567890');
UPDATE `test`
SET `field` = REGEXP_REPLACE(`field`, '[7]', 'z');
SELECT `field`
FROM `test`;

field

56z890