mariaDB extract only the digits from {"user":"128"} - regex

Question

From the string {"user":"128"}, expected only 128.
Could also be {"user":"8"} or {"user":"12"} or {"user":"128798"}
In this fiddle example the matched value should be 3
Schema:
CREATE TABLE test1 (
id INT AUTO_INCREMENT,
userid INT(11),
PRIMARY KEY (id)
);
INSERT INTO test1(userid)
VALUES
('126'),
('2457'),
('3'),
('40');
CREATE TABLE test2 (
id INT AUTO_INCREMENT,
code VARCHAR(1024),
PRIMARY KEY (id)
);
INSERT INTO test2(code)
VALUES
('{"user":"128"}'),
('{"user":"2459"}'),
('{"user":"3"}'),
('{"user":"46"}');
Query:
(SELECT sub.`userid`, rev.`code` REGEXP '[0-9]'
FROM `test1` AS sub, `test2` AS rev
WHERE sub.`userid`= rev.`code`
)
Have tried REGEXP '[0-9]' and also '[[:digit:]]' but no luck.
I have also tried concat ('',value * 1) = value or concat(value * 1) = value
SOLUTION: fiddle

Answer 1

REGEXP will return 0 or 1 if the regexp matches the string.
You should have a look to 'regexp_substr'
https://dev.mysql.com/doc/refman/8.0/en/regexp.html#function_regexp-substr
select REGEXP_SUBSTR(code, '"[0-9]+"') from test2
This works for me, after selecting MySQL 8 version in your fiddler.
it returns :
"128"
"2459"
null
"46"
or
select REGEXP_SUBSTR(code, '[0-9]+') from test2
if you want 3 instead of null for "k3"
akina suggestion of using JSON_EXTRACT is way better, but does require that you redefine the column as a JSON datatype from VARCHAR.

Answer 2

Use the function REGEXP_SUBSTR:
MariaDB [(none)]> SELECT REGEXP_SUBSTR('{"user":"128"}', '\\d+');
+-----------------------------------------+
| REGEXP_SUBSTR('{"user":"128"}', '\\d+') |
+-----------------------------------------+
| 128 |
+-----------------------------------------+
1 row in set (0.000 sec)