[{_id:1,val:"Abdominal Cramp"},{_id:2,val:"Agraphia"},{_id:3,val:"Afebrile"},{_id:4,val:"Cyclic Fever"},{_id:5,val:"Fever"},{_id:6,val:"High Fever"},{_id:7,val:"Continuous Fever"},]

db.collection.aggregate([{$match:{val:{$regex:"fe",$options:"i"}}},{"$addFields":{prefix:{"$cond":[{"$regexMatch":{"input":"$val","regex":"^fe","options":"i"}},1,0]}}},{$sort:{prefix:-1,val:1}},{$project:{prefix:0}}])