There is an example (https://www.zabbix.com/documentation/4.0/manual/web_monitoring/example):

Take note also of how we are getting the content of the {sid} variable (session ID) using a variable syntax with regular expression: regex:name="sid" value="([0-9a-z]{16})".

The correct regex is regex:sid=([0-9a-z]{16}). Please fix.