PREG_CAPTURE and group by - mariadb goes down

[bezdelnique]

Hello.
I have a problem with combination PREG_CAPTURE and group by

Type of field which I am working with: requestUri varchar(255) null

This query works fine

SELECT PREG_CAPTURE('~^(/[^/]+)~', requestUri) as requestUri
FROM `dict_log_error404`

All the rest goes Mariadb down. The only way to make it live is command killall -9 mysqld.

All I need is make to do count.

SELECT PREG_CAPTURE('~^(/[^/]+)~', requestUri) as requestUri, count(*) as cnt
FROM dict_log_error404
group by requestUri

My another try

SELECT PREG_CAPTURE('~^(/[^/]+)~', requestUri) as requestUri
FROM dict_log_error404
where requestUri regexp '^(/[^/]+)'
group by requestUri

.. and another one

select count(*) as cnt, requestUri from
(
   SELECT PREG_CAPTURE('~^(/[^/]+)~', requestUri) as requestUri
   FROM dict_log_error404
   where requestUri regexp '^(/[^/]+)'
) e
group by requestUri

Help me, please.
I like PRCE so much! ;)