Lookup and Merge Objects based on Regex search match between two collections

Question

I need to perform $lookup aggregate and merge objects based on regex match. One collection is having regex match pattern. So based on these patterns, need to perform search on another collection.

I have two collections. (1) messages and (2) regexes. Here is output of message collection.

[{_id: ObjectId(...), message: I had a dinner at McDonald}, 

{_id: ObjectId(...), message: I booked my ticket from TicketWorld},

{_id: ObjectId(...), message: I spent my weekend at Paris},

{_id: ObjectId(...), message: I went to Paris},

{_id: ObjectId(...), message: I like McDonald}]

Here is output of regexes collection.

[{_id: ObjectId(...), regexMatch: /mcdonald/gi, ratePoint:3}, 

{_id: ObjectId(...), regexMatch: /paris/gi, ratePoint:4},

{_id: ObjectId(...), regexMatch: /ticketwor/gi, ratePoint:2]

I tried with following code. But could not perform regex search and hence not able to merge the objects.

db.messages.aggregate([ 

   { 

      $lookup: { 

         from: "regexes", 

         localField: "message",    

         foreignField: "regexMatch",  

         as: "fromItems"

      }

   },

   {

      $replaceRoot: { newRoot: { $mergeObjects: [ { $arrayElemAt: [ "$fromItems", 0 ] }, "$$ROOT" ] } }

   }, 

   { $project: { fromItems: 0 } } 

])

Expected output is as follows:

[{_id: ObjectId(...), message: I had a dinner at McDonald, ratePoint: 3}, 

{_id: ObjectId(...), message: I booked my ticket from TicketWorld ,ratePoint: 2}, 

{_id: ObjectId(...), message: I spent my weekend at Paris, ratePoint: 4},

{_id: ObjectId(...), message: I went to Paris, ratePoint: 4},

{_id: ObjectId(...), message: I like McDonald, ratePoint: 3}]