WITH t AS
(
SELECT 'ABRBRBDS' v FROM dual
UNION ALL SELECT 'A' FROM dual
UNION ALL SELECT 'AR' FROM dual
UNION ALL SELECT 'AB' FROM dual
UNION ALL SELECT 'BR' FROM dual
UNION ALL SELECT 'R' FROM dual
UNION ALL SELECT 'BRB' FROM dual
UNION ALL SELECT 'BDS' FROM dual
UNION ALL SELECT 'ABDS' FROM dual
UNION ALL SELECT 'BRBRD' FROM dual
UNION ALL SELECT 'BRBRBD' FROM dual
UNION ALL SELECT 'BRBDS' FROM dual
UNION ALL SELECT 'BRBS' FROM dual
UNION ALL SELECT 'B' FROM dual
UNION ALL SELECT 'D' FROM dual
UNION ALL SELECT 'S' FROM dual
UNION ALL SELECT 'DS' FROM dual
UNION ALL SELECT 'RB' FROM dual
)
SELECT v
     , CASE WHEN REGEXP_LIKE(v, '^(A|B|AB)?(RB){0,}(D|DS)?$')
            THEN 'O' ELSE 'X' END ox
  FROM t
;