Using regex in MongoDB findOne()

The findOne() returns one document that satisfies the specified query criteria on the collection. Let us create a collection with documents −

> db.demo655.insertOne({subject:"MySQL"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea050254deddd72997713cc")
}
> db.demo655.insertOne({subject:"MongoDB"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea0502b4deddd72997713cd")
}
> db.demo655.insertOne({subject:"Java"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5ea050314deddd72997713ce")
}

Display all documents from a collection with the help of find() method −

> db.demo655.find();

This will produce the following output −

{ "_id" : ObjectId("5ea050254deddd72997713cc"), "subject" : "MySQL" }
{ "_id" : ObjectId("5ea0502b4deddd72997713cd"), "subject" : "MongoDB" }
{ "_id" : ObjectId("5ea050314deddd72997713ce"), "subject" : "Java" }

Following is the query to use regex in findOne() −

> db.demo655.findOne({subject:{$regex:/M/}});

This will produce the following output −

{ "_id" : ObjectId("5ea050254deddd72997713cc"), "subject" : "MySQL" }