Improve Case-Insensitive Regex Queries

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If you frequently run case-insensitive regex queries (utilizing the i option), you should create a case-insensitive index to support your queries. You can specify a collation on an index to define language-specific rules for string comparison, such as rules for lettercase and accent marks. A case-insensitive index greatly improves performance for case-insensitive queries.


Consider an employees collection with the following documents. This collection has no indexes besides the default _id index:

// employees collection

  "_id": 1,
  "first_name": "Hannah",
  "last_name": "Simmons",
  "dept": "Engineering"
  "_id": 2,
  "first_name": "Michael",
  "last_name": "Hughes",
  "dept": "Security"
  "_id": 3,
  "first_name": "Wendy",
  "last_name": "Crawford",
  "dept": "Human Resources"
  "_id": 4,
  "first_name": "MICHAEL",
  "last_name": "FLORES",
  "dept": "Sales"

If your application frequently queries the first_name field, you may want to run case-insensitive regex queries to more easily find matching names. Case-insensitive regex also helps match against differing data formats, as in the example above where you have first_names of both “Michael” and “MICHAEL”.

If a user searches for the string “michael”, the application may run the following query:

db.employees.find( { first_name: { $regex: /michael/i } } )

Since this query specifies the $regex option i, it is case-insensitive. The query returns the following documents:

{ "_id" : 2, "first_name" : "Michael", "last_name" : "Hughes", "dept" : "Security" }
{ "_id" : 4, "first_name" : "MICHAEL", "last_name" : "FLORES", "dept" : "Sales" }

Although this query does return the expected documents, case-insensitive regex queries with no index support are not very performant. To improve performance, you can create a case-insensitive index on the first_name field:

  { first_name: 1 },
  { collation: { locale: 'en', strength: 2 } }

When the strength field of an index’s collation document is 1 or 2, the index is case-insensitive. For a detailed description of the collation document and the different strength values, see Collation Document.

For the application to use this index, you must also specify the same collation document from the index in the query. You can remove the $regex operator from the previous find() method and instead utilize the newly created index:

db.employees.find( { first_name: "michael" } ).collation( { locale: 'en', strength: 2 } )


Do not use the $regex operator when using a case-insensitive index for your query. The $regex implementation is not collation-aware and cannot utilize case-insensitive indexes.

Learn More

  • To learn more about case-insensitive indexes with illustrative examples, see Case Insensitive Indexes.
  • To learn more about regex queries in MongoDB, see $regex.
  • MongoDB University offers a free course on optimizing MongoDB Performance. To learn more, see M201: MongoDB Performance.