SQL Query to extract all posts with unchecked checkboxes

Something like that?

-- [params]
-- int_list :categories

SELECT p.id post_id,
       t.id topic_id,
       c.id category_id,
       (regexp_matches(p.raw, '\[\s\]\s*[^]\r\n]+', 'g'))[1] AS html$excerpt
FROM posts p
JOIN topics t ON p.topic_id = t.id
JOIN categories c ON t.category_id = c.id
WHERE p.raw ~ '\[\s\]\s*[^]\r\n]+'
  AND c.id IN (:categories)

I might have misunderstood something, but I believe it would be a good starting point.