Cannot create trigger for zabbix key using regexp including []

I cannot create trigger

{Template_App_XXXXX_VPN:web.page.regexp[10.100.39.40,/cgi-bin/hm_hm.sh,,"<ReturnStatus>[^<]*</ReturnStatus>",50].str(Completed)}

=0 because of the square brackets in the regexp. It returns error "Incorrect symbol sequence in trigger expression.". When I strip out the square brackets, this error goes away but it, of course, does not exist.

This monitoring key works just fine. It's simply the trigger that won't allow me to create it.