String Validation (length, charset, fullmatch)

Whole-string validation checks global rules—length bounds, allowed charset, regex shape, checksum digits—not just per-character typing. Often combine len, all(...), and re.fullmatch for strict full-string patterns.

Why this shows up in the real world

Form APIs reject emails or phone strings that fail pattern plus length rules. License keys validate structured segments and check digits.

Core idea (explained for students)

re.fullmatch(r'[A-Z]{2}\d{4}', s) anchors entire string. Layer checks: fast rejects first (if len(s) != 6: return False), then heavier regex.

Try this in Python

import re


def valid_ticket(code: str) -> bool:
    return bool(re.fullmatch(r"[A-Z]{3}-\d{4}", code))


print(valid_ticket("ABC-1234"), valid_ticket("abc-1234"))

Common mistakes

• Using re.search when you meant full-string match—partial substrings slip through.
• Unicode categories: \d vs [0-9] differ for full Unicode digits.

Key takeaways

• Order checks cheapest to most expensive.
• Return structured errors (too_short, bad_char) for better UX when building APIs.