I'm trying to define an action in QGIS using on of my fields but I have some problems with its format.

I have a numeric string like '5' and I need to have a format like '005'. I've tried function `tostring("myfield")` but I am not able to define the format that I want.stri

Is there any way to do something like `tostring(myfield,"000")`?

What I'm looking for is a function like this one: <http://office.microsoft.com/en-us/excel-help/text-HP005209313.aspx?CTT=5&origin=HP005204211>