Quick Answer
The correct answer is `re.sub(r'\bcat\b', 'dog', s)`. This code works because the `\b` metacharacter in the regex pattern `r'\bcat\b'` asserts a word boundary at both the start and end of 'cat', ensuring that only the standalone whole word is matched and replaced—not substrings like the 'cat' inside 'category'. On the Certified Associate Python Programmer PCAP exam, this question tests your understanding of the `re` module and the critical distinction between literal string replacement and pattern-based replacement using word boundaries. A common trap is forgetting the raw string prefix `r`, which can cause backslash-related syntax errors, or using `re.sub('cat', 'dog', s)` without boundaries, which would incorrectly alter 'category' to 'dogeory'. To remember this, think of `\b` as "bookends" for your word: they keep the match from spilling into neighboring characters.
This PCAP practice question tests your understanding of strings. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A developer needs to replace all occurrences of 'cat' with 'dog' in a string, but only if 'cat' is a whole word (not part of 'category'). Which code achieves this?
re.sub(r'\bcat\b', 'dog', s)
Word boundaries ensure whole word match.
s.replace('cat', 'dog')
Why wrong: Replaces all 'cat' anywhere, including inside 'category'.
re.sub('cat', 'dog', s)
Why wrong: Replaces all 'cat' anywhere, no word boundary.
s.replace('cat', 'dog', 1)
Why wrong: Only replaces first occurrence.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
re.sub(r'\bcat\b', 'dog', s)
Option A uses the `re.sub()` function with the regex pattern `r'\bcat\b'`, where `\b` denotes a word boundary. This ensures that only the whole word 'cat' is matched and replaced with 'dog', ignoring cases where 'cat' appears as part of a larger word like 'category'. The `r` prefix makes it a raw string, preventing escape sequence issues.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
re.sub(r'\bcat\b', 'dog', s)
Why this is correct
Word boundaries ensure whole word match.
Related concept
Read the scenario before looking for a memorised answer.
s.replace('cat', 'dog')
Why it's wrong here
Replaces all 'cat' anywhere, including inside 'category'.
re.sub('cat', 'dog', s)
Why it's wrong here
Replaces all 'cat' anywhere, no word boundary.
s.replace('cat', 'dog', 1)
Why it's wrong here
Only replaces first occurrence.
Common exam traps
Python Institute often tests the distinction between simple string methods and regex-based substitution, specifically the need for word boundary anchors (`\b`) to match whole words, which candidates overlook when they assume `replace()` or a plain `re.sub()` pattern is sufficient.
Detailed technical explanation
The `\b` metacharacter in regex matches a zero-width position between a word character (`\w`) and a non-word character (`\W`), or at the start/end of the string. This is crucial for natural language processing tasks like tokenization or search-and-replace in text editors. In Python, raw strings (`r'...'`) are recommended for regex patterns to avoid double escaping backslashes, which can lead to subtle bugs.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
What to study next
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FAQ
Strings — This question tests Strings — Read the scenario before looking for a memorised answer..
The correct answer is: re.sub(r'\bcat\b', 'dog', s) — Option A uses the `re.sub()` function with the regex pattern `r'\bcat\b'`, where `\b` denotes a word boundary. This ensures that only the whole word 'cat' is matched and replaced with 'dog', ignoring cases where 'cat' appears as part of a larger word like 'category'. The `r` prefix makes it a raw string, preventing escape sequence issues.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
This PCAP practice question is part of Courseiva's free Python Institute certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCAP exam.
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