Lookup output to variable

Working with Data
1

I would like to output the result of a $lookup depending on the content of the “looked up” document. How can this be done?

$lookup: {
            from: "device_instances",
            let: { path_regex: {$concat: ["^", "$path"]}, device_category: { $concat: ["catecories.","$category"]}},
            pipeline: [{
                $match: {
                    $expr: {
                        $regexMatch: {
                            input: "$path",
                            regex: "$$path_regex",
                            options: "i"
                        }
                    }
                }
            }],
            as: "$$device_category"
        }