Zabbix trigger for finding difference in the previous and current item output

I have got an item, which executes some powershell script on remote machine, it returns string. I want to set zabbix trigger, to activate if current and previous output of script are different. I have tried something like this:

{MyTemplate:MyItem.diff()=1}

and also:

{MyItem.change(0)}#0

None of them seem to be working. Any ideas how to solve it?

Ok I solved this problem, turned out, that i have not connected host to template and he wasn’t sending any data back.

This solution turned out to be good, but not exactly what i wanted to achieve. So now I would like to create trigger which activates if item value is not empty. Because it returns some text, I thought it would be the best to execute it if length of item is greater than 0. But I can’t find this expression in documentation and I will really appreciate any help.