String Validation with Security and Bypass Prevention

Problem Overview

Design and implement a string validation function with security-oriented constraints. The function must correctly validate input strings according to defined rules (similar in algorithmic structure to a LeetCode Medium string-processing problem), but must also account for: (1) adversarial inputs crafted to bypass the validation logic, (2) performance and correctness…

• 4 candidate-reported follow-ups — the exact probes interviewers asked, with the trigger for each
• The rest of the problem statement — full requirements, constraints, and edge cases
• Approach and trade-offs — what passing candidates did, and the mistakes that sink people