Any pattern that is valid in PCRE2 should also be accepted by the Zabbix frontend.
However, the following valid PCRE2 expression is rejected by the frontend validator during preprocessing:

String: <div _ngcontent-ng-c20724544=\"\">0 kg</div>
Regex: <div _ngcontent-ng-c20724544=\\"\\">(\d+).*<\/div>

Note: On 7.0 it is working.