Regex queries in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents with names starting with 'A'

Given a collection users with documents containing a name field, which query returns all users whose names start with the letter 'A'?

MongoDB

db.users.find({ name: { $regex: '^A' } })

A db.users.find({ name: { $regex: '^A' } })

B db.users.find({ name: { $regex: 'A$' } })

C db.users.find({ name: { $regex: 'A' } })

D db.users.find({ name: { $regex: '^a' } })

💡 Hint

Use ^ to match the start of the string.

The caret ^ in regex means the start of the string. So ^A matches names starting with 'A'. Option A matches names ending with 'A'. Option A matches names containing 'A' anywhere. Option A matches names starting with lowercase 'a', which is case sensitive.

❓ query_result

intermediate

Find documents with emails ending in '.com'

Which MongoDB query finds all documents in contacts collection where the email field ends with '.com'?

MongoDB

db.contacts.find({ email: { $regex: '\.com$' } })

A db.contacts.find({ email: { $regex: '\.com' } })

B db.contacts.find({ email: { $regex: '^\.com' } })

C db.contacts.find({ email: { $regex: '.com' } })

D db.contacts.find({ email: { $regex: '\.com$' } })

💡 Hint

Use $ to match the end of the string and escape the dot.

The dollar sign $ matches the end of the string. The dot . is a special character in regex, so it must be escaped with a backslash \. to match a literal dot. Option D correctly matches emails ending with '.com'. Option D matches strings starting with '.com'. Option D matches any string containing 'com' with any character before it. Option D matches any string containing '.com' anywhere.

📝 Syntax

advanced

Identify the syntax error in regex query

Which of the following MongoDB queries has a syntax error and will fail to run?

A )} } '}4{d\}3{]Z-A[' :xeger$ { :edoc {(dnif.stcudorp.bd

B db.products.find({ code: { $regex: '[A-Z]{3}\d{4}' } })

C b.products.find({ code: { $regex: '[A-Z]{3}\d{4}' } })

D db.products.find({ code: { $regex: '[A-Z]{3}\d{4}' } }

💡 Hint

Check for missing brackets or braces.

Option B is missing the closing brace } for the find method call, causing a syntax error. The other options have correct syntax.

❓ optimization

advanced

Optimize regex query for case-insensitive search

You want to find all documents in employees where the department field contains the word 'sales' regardless of case. Which query is the most efficient and correct?

A db.employees.find({ department: { $regex: '(?i)sales' } })

B db.employees.find({ department: { $regex: '[Ss][Aa][Ll][Ee][Ss]' } })

C db.employees.find({ department: { $regex: 'sales', $options: 'i' } })

D db.employees.find({ department: { $regex: 'sales' } })

💡 Hint

Use regex options for case-insensitivity.

Option C uses the $options: 'i' flag to make the regex case-insensitive, which is efficient and clear. Option C tries to use inline case-insensitive flag which MongoDB does not support. Option C manually matches each letter case which is less efficient. Option C is case-sensitive and misses matches with different cases.

🧠 Conceptual

expert

Understanding regex anchoring in MongoDB queries

Consider the query db.logs.find({ message: { $regex: 'error' } }). Which statement best describes the behavior of this query?

A It matches documents where 'message' contains the substring 'error' anywhere.

B It matches documents where 'message' starts with 'error'.

C It matches documents where 'message' ends with 'error'.

D It matches documents where 'message' equals exactly 'error'.

💡 Hint

Think about what happens when no anchors are used in regex.

Without anchors like ^ or $, the regex matches the pattern anywhere in the string. So option A is correct. Options A and B require anchors. Option A requires exact match, which regex alone does not guarantee.