Unrecognized expression ‘$regex’ - regex

Question

Not able to get desired output. Getting “Unrecognized expression ‘$regex’” error
[
{
'$lookup': {
'from': 'profiles',
'let': {
'userId': '$userId',
},
'pipeline': [
{
'$match': {
$expr: {
$and: [
{ '$eq': ['$uniqueId', '$$mcontactId'] },
{
$or: [{ 'birthDate': { '$regex': '$$today' } },
{ 'spouseBirthdate': { '$regex': '$$today' } },
{ 'weddingAnniversary': { '$regex': '$$today' } },
],
},
],
},
},
},
{
'$project': {
'userId': 1,
'uniqueId': 1,
'mobileNumber': 1,
'whatsApp': 1,
'emailId': 1,
'lastName': 1,
'firstName': 1,
'address': 1,
'signature': 1,
},
},
],
'as': 'profile',
},
},
]

Answer 1

$regex is a query operator, you are trying to use it within an $expr which uses the "aggregation" language as oppose to the "query" language normally used within a $match stage.
Apart from that you have some other issue's in your pipeline, for example you only define $userId as a variable for the $lookup stage but in it you're trying to use $$today and $$mcontactId which are not defined anywhere.
Regardless once you sort out those issue's you have two options:
if the regex match is not related to the input variables just use $regex outside the $expr, like so:
{
'$match': {
$and: [
{
$expr: {
'$eq': [
'$uniqueId',
'$$userId',
],
},
},
{
$or: [
{
'birthDate': {
'$regex': '03-05',
},
},
],
},
],
},
},
Mongo Playground
if the regex does not to use an input variable from the $lookup then you need to use an aggregation operator, like $regexMatch to do the match within the $expr