How to use $regex in mongodb aggregation query within $match

Question

I am trying to use the $regex within $match, its not returning the matching documents.

db.collection('MyCollection', function (err, collection) {
  collection.aggregate([
    { $match: { 'Code': 'Value_01', 'Field2': { $regex: '/Value_2/g' } } },  
    { $project: {
        _id: 1,
        CodeNumber: '$Code',
        FieldName2: '$Field2'
      }
    }
  ], function (err, Result_doc) {
    console.log(Result_doc);
  }
});

Can anyone tell me where its going wrong or the correct syntax?

---

I even tried with replacing the

'Field2': { $regex: /Value_2/g }

Answer 1

As it says in the $regex docs you linked to, the two ways to do this are:

Field2: /Value_2/g

OR

Field2: { $regex: 'Value_2', $options: 'g' }

But I also tried your second attempt of 'Field2': { $regex: /Value_2/g } and that worked as well.

BTW, the g regex option doesn't make sense in this context as you just need one match anyway. Note that it isn't even listed in the $regex docs.

Answer 2

I got it working with the following code:

var Value_match = new RegExp('Value_2');

db.collection('MyCollection', function (err, collection) {

  collection.aggregate([
    { $match: { Code: 'Value_01', Field2: { $regex: Value_match } } },  
    { $project: {
        _id: 1,
        CodeNumber: '$Code',
        FieldName2: '$Field2'
      }
    }
  ], function (err, Result_doc) {
    console.log(Result_doc);
  }
});

On pushing the object content to console using console.dir(Value_match) it prints out '/Value_2/'