How can I use a regex variable in a query for MongoDB

Question

I would like to make query MongoDB for documents based on a regex expression that I contruct. For e.g I have constructed a simple regex as follows that is a combination of a

Answer 1

let test = `what your regex should 
search`;

collection.find({
    fieldName: {$regex: test}
}).then(result => red.send(result));

// tested on mongoose but should work on mongodb too