Maybe this can be useful to somebody.

preg_match('/[\w-.]+/', ''); // this will not work in PHP7.3 

preg_match('/[\w\-.]+/', ''); // the hyphen need to be escaped

You have to change the line 40 of /ow_utilities/validator.php to be able to use rss in widgets (and maybe elsewhere) again with php 7.3.

new line with hyphen escaped :

const URL_PATTERN = '/^(http(s)?:\/\/)?((\d+\.\d+\.\d+\.\d+)|(([\w\-]+\.)+([a-z,A-Z][\w\-]*)))(:[1-9][0-9]*)?(\/?([\w\-.\,\/:%+@&*=~]+[\w\- \,.\/?:%+@&=*|]*)?)?(#(.*))?$/';