On the screenshot, you should press "Add". Not very intuitive, but works.
Error message at the end, however, should not appear. Removed it in dev branch ZBX-3535

Perhaps a tester will want to know the commit that broke it. Could you please give a hint to him?

svn blame tr_logform.php

17571 artem $exprData = new CTriggerExpression(array('expression'=>construct_expression($itemid,$expressions)));

Situation when $expressions could be empty was not thought of in this case.

I propose we are more positive and use "svn praise" instead.

i do not want to create separate issue, so i write here.
How to reproduce:
1. For example open the triggers for the Template_Linux and open any trigger with DIFF function.
2. Toggle input method
3.Click to expression
4.Press Edit button
5.voila - in the Function field you see a incorrect function preselecting (is not the "diff >").
The same for other function - regexp, and maybe not only.

Oleksiy, this relates to https://support.zabbix.com/browse/ZBX-3454
I've reposted your comment there.

merged: branches/1.8 r17920:17922