Hackerrank Regular Expressions (re module)

Question 1

Which re function returns a match object only if the pattern matches at the BEGINNING of the string?

Accepted Answer

re.match()

Answer

re.match() anchors the pattern to the start of the string. re.search() scans the entire string for a match anywhere. re.fullmatch() requires the pattern to cover the entire string.

Question 2

What does re.findall(r'\d+', 'abc123def456') return?

Accepted Answer

['123', '456']

Answer

\d+ matches one or more consecutive digits. re.findall returns a list of all non-overlapping matches as strings. The two digit sequences '123' and '456' are found, giving ['123', '456'].

Question 3

What is the output of the following?

python
import re
m = re.search(r'(\w+)@(\w+)', 'user@domain.com')
print(m.group(1))

Accepted Answer

user

Answer

Parentheses in a regex create capturing groups numbered from 1. group(0) is the full match; group(1) is the first captured group (\w+ before @), which matches 'user'.

Question 4

What does re.sub(r'\s+', '-', 'hello   world	there') return?

Accepted Answer

'hello-world-there'

Answer

\s+ matches one or more whitespace characters (spaces, tabs, newlines). re.sub replaces every such sequence with '-', converting all whitespace runs to a single dash.

Question 5

Which pattern correctly matches a string that starts with a digit and ends with a letter (case-insensitive)?

Accepted Answer

r'^\d.*[a-zA-Z]$'

Answer

^ anchors to the start, \d matches one digit, .* allows any characters in between, [a-zA-Z] matches a letter, and $ anchors to the end. Together this enforces the full-string constraint.

(Hackerrank) HackerRank Python Certification Practice Test

(Hackerrank) HackerRank Python Certification Practice Test

Hackerrank Regular Expressions (re module)