how to programmatically decide if a menu item is selected or not

[moda20]

i am trying to make a menu show up as selected if the current location is equal to the href or if one of it's parts is.
so the item with href : /part1 should be selected if the current location is /part1 or /part1/part2.

currently this seems to not be possible as there is no way to use custom isCurrentURI functions or a regex based solution. is there another way of getting the desired outcome