Symphony.

I am trying to filter datasource using regex and URL param... The problem is when there is no my URL param is empty no results are shown... but I want to show all entries if no param is present. How can I achieve this?

Right now I just use: http://www.localhost.lan/blog/?tag=santaclaus

and filter like this:

regexp: {$url-tag}

Could you attach a screenshot of your datasource editor page?

Have a look: http://d.pr/oSJm

I would suggest a default fallback value, something like

$regex:{$url-tag:0}

Replace the 0 with whatever would suffice as a default value. Hopefully that will work.