Use $regex inside $expr in mongodb aggregation - regex

Question

My doc looks as follows
doc = {
name: 'abc',
age:20
}
and my query looks like
{ $expr: {$and:[{ $gt:[ "$age", 10 ] },
{ $regex:["$name",'ab']}
]
}
} }
But it's not working and I get an error
Unrecognized expression '$regex'
How can I make it work?
My original query looks like this
db.orders.aggregate([{
$match: {}},
{$lookup: {
from: "orders",
let: {
"customer_details": "$customerDetails"
},
pipeline: [
{
$match: {
$expr: {
$and: [
{ $or: [
{
$eq: ["$customerDetails.parentMobile","$$customer_details.parentMobile"]
},
{$eq: ["$customerDetails.studentMobile","$$customer_details.parentMobile"]
},
{$eq: ["$customerDetails.studentMobile","$$customer_details.parentMobile"]
},
{$eq: ["$customerDetails.studentMobile","$$customer_details.studentMobile"]
}
]
},
{$eq: ["$customerDetails.zipCode","$$customer_details.zipCode"]},
{$eq: ["$customerDetails.address","$$customer_details.address"]}
]
}
}
}],
as: "oldOrder"
}
}])
I want to use regex for matching address.
Any help will be greatly appreciated. Thanks in advance.

Answer 1

If your mongoDB version is 4.2, then you can use $regexMatch
try this
db.collection.find({
$expr: {
$and: [
{
$gt: [
"$age",
10
]
},
{
$regexMatch: {
input: "$name",
regex: "ab"
}
}
]
}
})
check this Mongo Playground

Answer 2

$regex is a query operator you cannot use inside $expr because it only supports aggregation pipeline operators.
{
"$expr": { "$gt": ["$age", 10] } ,
"name": { "$regex": "ab" }
}
If you have mongodb 4.2, you can use $regexMatch
{ "$expr": {
"$and": [
{ "$gt": ["$age", 10] },
{
"$regexMatch": {
"input": "$name",
"regex": "ab", //Your text search here
"options": "i",
}
}
]
}}