$addField by regex condition with Mongo < 4.2 - regex

Question

I have a mongoDB collection with the following objects:
{
name: "a",
email: "aaa#bestemail.com"
},
{
name: "b",
email: "bbb#bestemail123.com"
},
{
name: "c",
email: "ccc#diffemail.com"
}
I want to add to each document a field of true/false if the email address matches the /bestemail/ regex expression.
Meaning, for the current collection I would get:
{
name: "a",
email: "aaa#bestemail.com",
matching: true
},
{
name: "b",
email: "bbb#bestemail123.com",
matching: true
},
{
name: "c",
email: "ccc#diffemail.com",
matching: false
}
The main issue is that I have to work with mongo version 4.0.12 and so I cannot use $regexMatch (available from mongo 4.2)
I have tried using:
$addFields: {"matching": {$cond: [{$in: ["$email", [/bestemail/]]}, true, false]}}
but I get matching: false for all documents.
Thank you for the assistance!

Answer 1

I hesitate to respond since I don't have access to a MongoDB server with version 4.0.12, but perhaps this will work. [I did look at the archived docs.]
db.collection.aggregate([
{
"$addFields": {
"matching": {
"$cond": [
{
"$gt": [
{ "$indexOfCP": [ "$email", "bestemail" ] },
0
]
},
true,
false
]
}
}
}
])
Try it on the wrong server version at mongoplayground.net.
Example output:
[
{
"_id": ObjectId("5a934e000102030405000000"),
"email": "aaa#bestemail.com",
"matching": true,
"name": "a"
},
{
"_id": ObjectId("5a934e000102030405000001"),
"email": "bbb#bestemail123.com",
"matching": true,
"name": "b"
},
{
"_id": ObjectId("5a934e000102030405000002"),
"email": "ccc#diffemail.com",
"matching": false,
"name": "c"
}
]