Regular expressions for text cleaning in NLP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this regex substitution?

Given the text "Hello!!! Are you #1?", what is the result after applying re.sub(r'[^a-zA-Z0-9 ]', '', text)?

NLP

import re
text = "Hello!!! Are you #1?"
result = re.sub(r'[^a-zA-Z0-9 ]', '', text)
print(result)

A Hello Are you 1?

B Hello!!! Are you 1

C Hello Are you #1

D Hello Are you 1

💡 Hint

The regex removes all characters except letters, digits, and spaces.

The pattern [^a-zA-Z0-9 ] matches any character that is NOT a letter, digit, or space. So punctuation like '!!!', '#', and '?' are removed.

❓ Model Choice

intermediate

Which regex pattern removes all digits from a string?

You want to remove all digits from a text string using re.sub. Which pattern should you use?

A r'\d+'

B r'\D+'

C r'\w+'

D r'\s+'

💡 Hint

Digits are represented by \d in regex.

The pattern \d+ matches one or more digits. Using re.sub(r'\d+', '', text) removes all digits.

❓ Metrics

advanced

How many tokens remain after cleaning?

Given the text "Data science 101: Clean, analyze, & visualize!", after applying re.sub(r'[^a-zA-Z ]', '', text).lower().split(), how many tokens are in the resulting list?

NLP

import re
text = "Data science 101: Clean, analyze, & visualize!"
cleaned = re.sub(r'[^a-zA-Z ]', '', text).lower().split()
print(len(cleaned))

A 7

B 5

C 6

D 4

💡 Hint

Digits and punctuation are removed before splitting by spaces.

After removing non-letters and spaces, '101', ':', ',', '&', and '!' are removed. The remaining words are ['data', 'science', 'clean', 'analyze', 'visualize'], totaling 5 tokens.

🔧 Debug

advanced

Why does this regex fail to remove punctuation?

This code aims to remove punctuation but does not work as expected:

import re
text = "Hello, world!"
cleaned = re.sub(r'[\w]', '', text)
print(cleaned)

Why?

NLP

import re
text = "Hello, world!"
cleaned = re.sub(r'[\w]', '', text)
print(cleaned)

A The pattern '[\w]' matches letters and digits, so it removes them instead of punctuation.

B The pattern '[\w]' matches punctuation only, so letters remain.

C The pattern is missing a quantifier like '+' to match multiple characters.

D The pattern should be '[^\w]' to remove punctuation.

💡 Hint

Check what \w matches in regex.

The pattern '[\w]' matches any letter, digit, or underscore. So it removes letters and digits, leaving punctuation untouched.

🧠 Conceptual

expert

Which regex pattern best cleans URLs from text?

You want to remove URLs from text data using regex. Which pattern is most effective?

A r'http://'

B r'www\.\w+'

C r'https?://\S+'

D r'\S+\.com'

💡 Hint

URLs often start with http or https and continue until a space.

The pattern https?://\S+ matches 'http://' or 'https://' followed by non-space characters, effectively capturing full URLs.