[{str:"https://media.test.com/$main$\n \n https://media.xl?$desktop$\n \n https://media.com/$desktop/\n \n"},{}]

db.collection.aggregate([{$match:{$expr:{$eq:[{$type:"$str"},"string"]}}},{$project:{newString:{$substr:["$str",0,{$indexOfBytes:["$str","\n"]}]}}}])