db={"client":[{name:"Thomas Smith",card_type:"Gold",card_term:new Date("2015-05-03"),gender:"male",password:"dsfsfds",phone_number:"494666027550"}],"employee":[{name:"Patrick Manson",employee_datebirth:new Date("1986-05-08"),employment_start:"2014-12-06",gender:"male",paycheck:800,phone_number:"497185535712",position:"bodyweight-trainer"}]}

db.client.aggregate([{$lookup:{from:"employee",let:{c_phone_number:"$phone_number"},pipeline:[{$addFields:{tmp:"$$c_phone_number"}},{$match:{phone_number:{$regex:".*55.*"},tmp:{$regex:".*55.*"}}},{$project:{tmp:0}}],as:"result"}},{$match:{"result.0":{$exists:true}}}])