The correct approach is `re.sub(r'[0-9]', 'X', message)`, which uses Python’s `re.sub` function with a character class `[0-9]` to match any single digit and replace it with 'X'. This works because the regex pattern `[0-9]` defines a range that includes all digits 0 through 9, and `re.sub` iterates through the string, replacing every match individually—making it ideal for data sanitization where every digit must be masked. On the Certified Associate Python Programmer PCAP exam, this question tests your understanding of the `re` module’s substitution method versus common pitfalls like using `str.replace()` (which only handles literal strings, not patterns) or incorrect regex syntax like `\d+` (which would replace entire multi-digit sequences with a single 'X', not each digit separately). A frequent trap is confusing `re.sub` with `str.replace` or forgetting that `[0-9]` matches one character at a time. Memory tip: think of `[0-9]` as a “digit-by-digit” scanner—each digit gets its own 'X', so “123” becomes “XXX”.
PCAP Strings Practice Question
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 logging module receives a message that may contain sensitive data. To comply with data privacy, all digits in the message should be replaced with 'X' before logging. Which approach correctly achieves this?
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'[0-9]', 'X', message)
Option B is correct because `re.sub(r'[0-9]', 'X', message)` uses a regular expression character class `[0-9]` to match any single digit (0-9) and replaces each occurrence with 'X'. This is the standard Python approach for pattern-based string replacement, and it correctly handles all digits without affecting non-digit characters.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
message.replace('0-9', 'X')
Why it's wrong here
replace() treats the first argument as a literal string, not a character range.
✓
re.sub(r'[0-9]', 'X', message)
Why this is correct
Uses regex to replace any digit character with 'X'.
Related concept
Read the scenario before looking for a memorised answer.
This would also work, but requires manual mapping; re.sub is more concise.
✗
''.join(['X' if c.isdigit() else c for c in message])
Why it's wrong here
This works but is less efficient and not a built-in method.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Python Institute often tests the distinction between literal string methods (`str.replace()`) and pattern-based methods (`re.sub()`), trapping candidates who assume `replace()` can interpret character ranges like `'0-9'` as a regex pattern.
Detailed technical explanation
How to think about this question
The `re.sub()` function from Python's `re` module compiles the pattern `[0-9]` into an internal state machine that scans the string linearly, matching each digit and replacing it with 'X' in a single pass. This is more efficient than iterating character-by-character in pure Python (as in Option D) for large strings, because the regex engine is implemented in C. In real-world logging systems, such as those using Python's `logging` module with custom filters, `re.sub()` is commonly used to sanitize sensitive data like credit card numbers or social security numbers before writing to logs.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Strings — This question tests Strings — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: re.sub(r'[0-9]', 'X', message) — Option B is correct because `re.sub(r'[0-9]', 'X', message)` uses a regular expression character class `[0-9]` to match any single digit (0-9) and replaces each occurrence with 'X'. This is the standard Python approach for pattern-based string replacement, and it correctly handles all digits without affecting non-digit characters.
What should I do if I get this PCAP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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