Hackerrank Regular Expressions (re module) — Questions and Answers
Question 1: Which re function returns a match object only if the pattern matches at the BEGINNING of the string?
- re.match() (Correct answer)
- re.search()
- re.fullmatch()
- re.findall()
Correct answer: re.match()
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?
- ['123', '456'] (Correct answer)
- ['1', '2', '3', '4', '5', '6']
- [123, 456]
- '123'
Correct answer: ['123', '456']
\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))
- user (Correct answer)
- user@domain
- domain
- user@domain.com
Correct answer: user
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\tthere') return?
- 'hello-world-there' (Correct answer)
- 'hello world\tthere'
- 'hello-world\tthere'
- 'hello - world - there'
Correct answer: 'hello-world-there'
\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)?
- r'^\d.*[a-zA-Z]$' (Correct answer)
- r'\d.*[a-zA-Z]'
- r'^[0-9][a-z]$'
- r'\d+[a-zA-Z]+'
Correct answer: r'^\d.*[a-zA-Z]$'
^ 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.
Question 6: What is the purpose of re.compile() compared to using re.match() directly?
- It pre-compiles the pattern into a reusable regex object, improving performance when the same pattern is used many times (Correct answer)
- It validates that the regex pattern is syntactically correct without matching anything
- It converts the pattern to a case-insensitive version automatically
- It is required before any regex function can be called in Python
Correct answer: It pre-compiles the pattern into a reusable regex object, improving performance when the same pattern is used many times
re.compile(pattern) parses and compiles the regex into a Pattern object. Reusing this object avoids recompiling on every call, which is beneficial in loops or repeated matching. Using re.match() directly recompiles internally each time.
Hackerrank Regular Expressions (re module)
Which re function returns a match object only if the pattern matches at the BEGINNING of the string?