MySQL simple query to extract substring given 2 delimiters - mysql

Question

Specifically I need to pick the part of field label from table tmp.label between delimiters <!-- rwbl_1 --> and <!-- rwbl_2 --> where label contains <span in order to be able to update that field for records erroneously formatted with HTML tags.
The result amounts to something like strip_tags. This is, obviously, only possible due to the presence of the abovementioned (or similar) delimiters.

Answer 1

Here is a simple solution for the abovementioned case to extract a substring given 2 delimiters.
SELECT
SUBSTRING_INDEX(
SUBSTRING_INDEX(label,
'<!-- rwbl_1 -->', -1),
'<!-- rwbl_2 -->', 1) AS cutout
FROM tmp.label
WHERE label LIKE '<span%'
ORDER BY 1
LIMIT 1;
More abstract:
SELECT
SUBSTRING_INDEX(
SUBSTRING_INDEX(string_field, # field name
'delimiter_1', -1), # take the right part of original
'<!-- delimiter_2 -->', 1) # take the left part of the resulting substring
AS cutout
FROM my_table
WHERE my_condition;
And the update now works like this:
UPDATE my_table
SET string_field = SUBSTRING_INDEX(
SUBSTRING_INDEX(string_field,
'delimiter_1', -1),
'<!-- delimiter_2 -->', 1)
WHERE my_condition;