MariaDB 10.4

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id	C1
1	[AU 1] string 1; [AU 2] string 2; [AU 3] string 3.1; string 3.2; [AU 4] string 4.1; string 4.2; [AU 5] string 5
2	[AU 1; AU 2] string 1
3	[AU 1] string 1; [AU 2] string 2

WITH RECURSIVE
cte1 AS ( SELECT id, 
                 TRIM(TRAILING ';' FROM TRIM(SUBSTRING_INDEX(C1, '[', 2))) one_group,
                 SUBSTRING(C1 FROM LENGTH(SUBSTRING_INDEX(C1, '[', 2))) slack,
                 1 ordinality_au
          FROM test
          UNION ALL
          SELECT id, 
                 TRIM(TRAILING ';' FROM TRIM(SUBSTRING_INDEX(slack, '[', 2))),
                 SUBSTRING(slack FROM LENGTH(SUBSTRING_INDEX(slack, '[', 2))),
                 ordinality_au + 1
          FROM cte1
          WHERE LOCATE('[', slack) ),
cte2 AS ( SELECT id,
                 CONCAT(SUBSTRING_INDEX(one_group, ']', 1), ']') AU,
                 TRIM(SUBSTRING_INDEX(one_group, ']', -1)) ORG,
                 ordinality_au
          FROM cte1 ),
cte3 AS ( SELECT id, 
                 AU,
                 ordinality_au,
                 SUBSTRING_INDEX(ORG, ';', 1) ORG,
                 TRIM(TRIM(LEADING ';' FROM TRIM(LEADING SUBSTRING_INDEX(ORG, ';', 1) FROM ORG))) slack,
                 1 ordinality_org
          FROM cte2 
          UNION ALL
          SELECT id, 
                 AU,
                 ordinality_au,
                 SUBSTRING_INDEX(slack, ';', 1),
                 TRIM(TRIM(LEADING ';' FROM TRIM(LEADING SUBSTRING_INDEX(slack, ';', 1) FROM slack))),
                 ordinality_org + 1
          FROM cte3
          WHERE TRIM(slack) != '' )
SELECT id,
       AU, 
       ORG
FROM cte3
ORDER BY id, ordinality_au, ordinality_org;

id	AU	ORG
1	[AU 1]	string 1
1	[AU 2]	string 2
1	[AU 3]	string 3.1
1	[AU 3]	string 3.2
1	[AU 4]	string 4.1
1	[AU 4]	string 4.2
1	[AU 5]	string 5
2	[AU 1; AU 2]	string 1
3	[AU 1]	string 1
3	[AU 2]	string 2