The regex part itself wasn't too bad but the follow-up about counting distinct valid formats is where I started fumbling.
Start by clarifying the exact set of valid formats and delimiters with the interviewer, then build the regex incrementally, testing each component. Finally, count the distinct formats your regex accepts and discuss any trade-offs between strictness and flexibility.
Pro tip: Demonstrate awareness of edge cases like optional parentheses, multiple delimiter types, and the ambiguity of 'other delimiters'—ask clarifying questions to avoid over-engineering. Also, mention that counting distinct formats depends on how you define 'format' (e.g., delimiter choices and optional components).
Ask the interviewer to specify allowed delimiters (hyphens, spaces, dots?), whether parentheses are only around area code, and if extensions are allowed. This prevents assumptions that could lead to an incorrect regex.
Build the regex piece by piece: start with area code (optional parentheses), then central office code, then line number, allowing delimiters between groups. Use non-capturing groups and character classes for delimiters.
Mentally or verbally test the regex against valid and invalid phone numbers to ensure it matches only the intended formats. Mention using tools like regex101 for verification.
Enumerate the combinations of optional parentheses and delimiter choices to determine how many distinct valid formats the regex accepts. Explain your counting method clearly.
Talk about the balance between flexibility and strictness, and how the regex might be adjusted for different use cases (e.g., international numbers).
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.