MariaDBに格納されているimgタグをいったんaタグで囲みつつ、もし二重にaタグで囲んでしまった場合には一組のタグを残して削除するためのテスト用のSQL文。

regexp_result_good.txt

MariaDB [(none)]> select regexp_replace(
  regexp_replace('<img src="https://pandanote.info/wordpress/wp-content/uploads/2018/12/P_20181201_191300_vHDR_Auto_a-233x300.jpg" alt="" width="233" height="300" class="alignnone size-medium wp-image-3517" />',
                 '(<img src=.*wp-image-(\\d+)[^>]*>)',
                 '<a href="https://pandanote.info/?attachment_id=\\2" rel="attachment wp-att-\\2">\\1</a>'),
'(?:(<a href[^>]*>))+(<img[^>]*>)(</a>)+',
'\\1\\2\\3');
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| regexp_replace(regexp_replace('<img src="https://pandanote.info/wordpress/wp-content/uploads/2018/12/P_20181201_191300_vHDR_Auto_a-233x300.jpg" alt="" width="233" height="300" class="alignnone size-medium wp-image-3517" />','(<img src=.*wp-image-(\\d+)[^>]                      |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| <a href="https://pandanote.info/?attachment_id=3517" rel="attachment wp-att-3517"><img src="https://pandanote.info/wordpress/wp-content/uploads/2018/12/P_20181201_191300_vHDR_Auto_a-233x300.jpg" alt="" width="233" height="300" class="alignnone size-medium wp-image-3517" /></a> |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+