SELECT td_url,
  -- 'http' と 'docs.fluentd.org' を入れ替え --
  -- $1=http, $2=//, $3=docs.fluentd.org --
  regexp_replace(td_url,'(http)(.*)(docs.fluentd.org)','$3$2$1') AS replace1
FROM access_log
WHERE regexp_like(td_url,'docs.fluentd.org')
LIMIT 10