Regex queries in MongoDB - Mini Project: Build & Apply

Regex Queries in MongoDB

📖 Scenario: You work at a small bookstore that stores book information in a MongoDB database. You want to find books by searching for patterns in their titles using regular expressions.

🎯 Goal: Build a MongoDB query that uses regex to find books with titles matching specific patterns.

📋 What You'll Learn

Create a collection called books with sample book documents

Add a regex pattern variable to search titles

Write a MongoDB query using $regex to find matching books

Complete the query with options to make the search case-insensitive

💡 Why This Matters

🌍 Real World

Bookstores and libraries often need to search for books by partial title matches or patterns. Regex queries in MongoDB help find these books efficiently.

💼 Career

Knowing how to use regex queries in MongoDB is useful for database developers and data analysts who work with flexible text search requirements.

1

Create the books collection with sample data

Create a MongoDB collection called books and insert these exact documents: { title: "The Great Gatsby", author: "F. Scott Fitzgerald" }, { title: "Great Expectations", author: "Charles Dickens" }, and { title: "Gone with the Wind", author: "Margaret Mitchell" }.

MongoDB

// Insert the specified book documents into the books collection

Hint

Use insertMany on db.books with an array of the three book objects.

2

Add a regex pattern variable for searching titles

Create a variable called pattern and set it to the regex string "Great" to search for titles containing the word "Great".

MongoDB

db.books.insertMany([
  { title: "The Great Gatsby", author: "F. Scott Fitzgerald" },
  { title: "Great Expectations", author: "Charles Dickens" },
  { title: "Gone with the Wind", author: "Margaret Mitchell" }
])

// Create the regex pattern variable

Hint

Use const pattern = "Great" to define the regex string.

3

Write a MongoDB query using $regex to find matching books

Write a MongoDB query that finds all documents in books where the title field matches the regex pattern stored in pattern. Use $regex with the variable pattern.

MongoDB

db.books.insertMany([
  { title: "The Great Gatsby", author: "F. Scott Fitzgerald" },
  { title: "Great Expectations", author: "Charles Dickens" },
  { title: "Gone with the Wind", author: "Margaret Mitchell" }
])

const pattern = "Great"

// Write the query to find books with titles matching the regex pattern

Hint

Use db.books.find({ title: { $regex: pattern } }) to query with regex.

4

Complete the query with case-insensitive option

Modify the query to add the $options field with value "i" to make the regex search case-insensitive.

MongoDB

db.books.insertMany([
  { title: "The Great Gatsby", author: "F. Scott Fitzgerald" },
  { title: "Great Expectations", author: "Charles Dickens" },
  { title: "Gone with the Wind", author: "Margaret Mitchell" }
])

const pattern = "Great"

db.books.find({ title: { $regex: pattern } })

// Add the $options: "i" to make the search case-insensitive

Hint

Add $options: "i" inside the title query object to ignore case.

Practice

1. What does the $regex operator do in a MongoDB query?

easy

A. It searches for documents where a field matches a text pattern.

B. It sorts documents in ascending order.

C. It updates documents with new values.

D. It deletes documents matching a condition.

Solution

1. Step 1: Understand the purpose of $regex

   The $regex operator is used to find text patterns inside string fields in MongoDB documents.

2. Step 2: Compare with other operations

   Sorting, updating, and deleting are different operations and do not use $regex.

3. Final Answer:

   It searches for documents where a field matches a text pattern. -> Option A

4. Quick Check:

   $regex = pattern search [OK]

Hint: Remember: $regex means 'match pattern' in text fields [OK]

Common Mistakes:

• Confusing $regex with sorting or updating operations
• Thinking $regex modifies data instead of searching
• Assuming $regex works on non-string fields

2. Which of the following is the correct syntax to find documents where the field name starts with 'A' using regex in MongoDB?

easy

A. {"name": {"$regex": "A$"}}

B. {"name": {"$regex": "A*"}}

C. {"name": {"$regex": "^A"}}

D. {"name": {"$regex": "[A]"}}

Solution

1. Step 1: Understand regex anchors

   The caret ^ means the start of the string, so ^A matches strings starting with 'A'.

2. Step 2: Check other options

   A$ matches strings ending with 'A', A* matches zero or more 'A's anywhere, and [A] matches any 'A' character anywhere.

3. Final Answer:

   {"name": {"$regex": "^A"}} -> Option C

4. Quick Check:

   Start with 'A' = ^A [OK]

Hint: Use ^ at start of pattern to match beginning of string [OK]

Common Mistakes:

• Using $ instead of ^ to match start
• Misunderstanding * as 'starts with'
• Using character class [] without anchors

3. Given the collection documents: [{"city": "London"}, {"city": "london"}, {"city": "Londonderry"}], what will the query {"city": {"$regex": "^London$", "$options": "i"}} return?

medium

A. []

B. [{"city": "London"}]

C. [{"city": "Londonderry"}]

D. [{"city": "London"}, {"city": "london"}]

Solution

1. Step 1: Analyze the regex pattern and options

   The pattern ^London$ matches exactly 'London' from start to end. The option i makes it case-insensitive.

2. Step 2: Check which documents match

   "London" matches exactly, "london" matches ignoring case, but "Londonderry" does not match because it has extra characters.

3. Final Answer:

   [{"city": "London"}, {"city": "london"}] -> Option D

4. Quick Check:

   Exact match ignoring case = [{"city": "London"}, {"city": "london"}] [OK]

Hint: Use ^ and $ for exact match; 'i' option ignores case [OK]

Common Mistakes:

• Ignoring case option and missing lowercase match
• Assuming partial matches without anchors
• Confusing $regex with substring search

4. Identify the error in this MongoDB query to find documents where title contains 'book' case-insensitive: {"title": {"$regex": "/book/i"}}

medium

A. The $regex operator cannot be used with strings.

B. The regex pattern should not be enclosed in slashes.

C. The $options field is missing.

D. The field name should be $pattern, not $regex.

Solution

1. Step 1: Understand MongoDB regex syntax

   In MongoDB, the regex pattern is a string without enclosing slashes. Options like 'i' go in a separate $options field.

2. Step 2: Identify the mistake in the query

   The pattern "/book/i" incorrectly includes slashes and option inside the string. It should be {"$regex": "book", "$options": "i"}.

3. Final Answer:

   The regex pattern should not be enclosed in slashes. -> Option B

4. Quick Check:

   MongoDB regex no slashes, use $options [OK]

Hint: Do not use /slashes/ in MongoDB regex patterns [OK]

Common Mistakes:

• Including slashes around regex pattern
• Putting options inside the pattern string
• Using wrong field names like $pattern

5. You want to find all documents where the description field contains either 'cat' or 'dog' (case-insensitive). Which MongoDB query correctly uses regex to achieve this?

hard

A. {"description": {"$regex": "cat|dog", "$options": "i"}}

B. {"description": {"$regex": "[cat|dog]", "$options": "i"}}

C. {"description": {"$regex": "(cat|dog)", "$options": "g"}}

D. {"description": {"$regex": "cat|dog"}}

Solution

1. Step 1: Understand regex alternation and options

   The pattern cat|dog matches either 'cat' or 'dog'. The i option makes it case-insensitive.

2. Step 2: Evaluate each option

   {"description": {"$regex": "cat|dog", "$options": "i"}} correctly uses the pattern and case-insensitive option. {"description": {"$regex": "[cat|dog]", "$options": "i"}} uses character class incorrectly, matching any single character 'c', 'a', 't', '|', 'd', 'o', or 'g'. {"description": {"$regex": "(cat|dog)", "$options": "g"}} uses g which is not a valid MongoDB regex option. {"description": {"$regex": "cat|dog"}} misses the case-insensitive option.

3. Final Answer:

   {"description": {"$regex": "cat|dog", "$options": "i"}} -> Option A

4. Quick Check:

   Use | for OR and 'i' for ignore case [OK]

Hint: Use | inside regex and $options: 'i' for case-insensitive [OK]

Common Mistakes:

• Using character classes [] instead of alternation |
• Using invalid regex options like 'g' in MongoDB
• Forgetting $options for case-insensitive search