Anthropic·Software Engineer·Technical Phone Screen·Senior

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May 2026Remote

Summary

Anthropic coding screen for a software engineer role, one question the whole time: build a string tokenizer from scratch. Seemed straightforward but the edge cases kept piling up and I ended up spending way more time on quoted string handling than I expected.

Questions Asked (1)

Implement a string tokenizer that splits an input string by a configurable delimiter set, handles quoted substrings as single tokens, supports backslash escaping inside quotes, and ignores empty tokens from consecutive or boundary delimiters.

Algorithms & Data StructuresTechnical Trade-offsAPI & Integrations

Author's notes

I started confident and immediately got tripped up on the quote handling.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the exact tokenization rules (delimiter set, quote characters, escape behavior, empty token handling) and edge cases. Then outline a single-pass state machine with states for normal, in-quote, and escape, and discuss trade-offs like performance, memory, and API design. Finally, walk through a few examples to validate the logic.

Pro tip: Mention that you would write comprehensive unit tests covering edge cases like empty input, only delimiters, unmatched quotes, and escaped quotes, and that you'd consider using a well-tested library if the requirements are standard, but implement custom logic if specific behaviors are needed.

1. Clarify requirements and edge cases

Ask about the delimiter set (e.g., comma, space), quote characters (single/double), escape character (backslash), and behavior for unmatched quotes or trailing backslashes. Confirm that empty tokens are ignored.

2. Design the state machine

Define states: NORMAL (accumulating token, checking for delimiters/quotes), IN_QUOTE (accumulating token, checking for closing quote/escape), ESCAPE (next character is literal). Specify transitions and actions (e.g., append to token, emit token).

3. Implement the tokenizer

Iterate through the string character by character, maintaining current state and a token buffer. On delimiter in NORMAL, emit token if non-empty and reset. On quote in NORMAL, enter IN_QUOTE. In IN_QUOTE, handle escape and closing quote. At end, emit any remaining token if non-empty.

4. Test with examples and edge cases

Walk through examples: 'a,b,c' -> [a,b,c]; 'a,,b' -> [a,b]; 'a,"b,c",d' -> [a, b,c, d]; 'a,"b\"c",d' -> [a, b"c, d]; '""' -> [] (empty token ignored). Discuss handling of unmatched quotes (e.g., treat as literal or error).

5. Discuss trade-offs and extensions

Compare single-pass O(n) time and O(n) space (for output) vs. regex or split-based approaches. Mention configurability (delimiter set, quote chars, escape char) and potential API design (e.g., function signature, options object).

Key Points to Mention

State machine approach for clarity and correctness
Handling of escape sequences inside quotes (backslash escaping)
Ignoring empty tokens from consecutive or boundary delimiters
Time and space complexity (O(n) time, O(n) space for output)
Edge cases: empty input, unmatched quotes, escaped quotes, multiple delimiters
Configurability: delimiter set, quote characters, escape character
Trade-offs: custom implementation vs. using standard library or regex

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.