How to use $regex in MongoDB?

The $regex operator in MongoDB allows you to perform pattern matching on string fields using regular expressions. It's useful for text searches, filtering data based on patterns, and implementing case-insensitive queries.

Syntax

db.collection.find({
    fieldName: { 
        $regex: "pattern",
        $options: "options" 
    }
});

Common options include i for case-insensitive matching, m for multiline, and x for extended syntax.

Sample Data

db.regularExpressionDemo.insertMany([
    {"UserName": "John"},
    {"UserName": "JOHN"},
    {"UserName": "john"},
    {"UserName": "JoHn"}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5cdffc25bf3115999ed51210"),
        ObjectId("5cdffc2ebf3115999ed51211"),
        ObjectId("5cdffc35bf3115999ed51212"),
        ObjectId("5cdffc3ebf3115999ed51213")
    ]
}

View Sample Data

db.regularExpressionDemo.find();

{ "_id": ObjectId("5cdffc25bf3115999ed51210"), "UserName": "John" }
{ "_id": ObjectId("5cdffc2ebf3115999ed51211"), "UserName": "JOHN" }
{ "_id": ObjectId("5cdffc35bf3115999ed51212"), "UserName": "john" }
{ "_id": ObjectId("5cdffc3ebf3115999ed51213"), "UserName": "JoHn" }

Example 1: Case-Sensitive Pattern Matching

db.regularExpressionDemo.find({'UserName': { $regex: 'JOHN'}});

{ "_id": ObjectId("5cdffc2ebf3115999ed51211"), "UserName": "JOHN" }

Example 2: Case-Insensitive Pattern Matching

db.regularExpressionDemo.find({'UserName': { $regex: 'JOHN', $options: 'i' }});

{ "_id": ObjectId("5cdffc25bf3115999ed51210"), "UserName": "John" }
{ "_id": ObjectId("5cdffc2ebf3115999ed51211"), "UserName": "JOHN" }
{ "_id": ObjectId("5cdffc35bf3115999ed51212"), "UserName": "john" }
{ "_id": ObjectId("5cdffc3ebf3115999ed51213"), "UserName": "JoHn" }

Common Patterns

• ^pattern - Matches strings starting with pattern
• pattern$ - Matches strings ending with pattern
• .*pattern.* - Matches strings containing pattern anywhere
• [aeiou] - Matches any single vowel character

Conclusion

The $regex operator provides powerful pattern matching capabilities in MongoDB. Use the $options parameter with value 'i' for case-insensitive searches, making it ideal for flexible text filtering and search functionality.