Character Validation (Rules, Regex-Style Logic, Guards)

Validation composes rules: length, allowed charset, no spaces, alternating patterns, etc. Think of it as boolean predicates you can unit-test independently before wiring into forms.

Why this shows up in the real world

Passport MRZ lines validate checksum characters per position. Coupon codes reject ambiguous letters (0 vs O) by design.

Core idea (explained for students)

Structure: def ok(s): return cond1(s) and cond2(s). Short-circuiting and stops early—order cheap checks first. For regex-heavy rules, compile patterns once with re.compile.

Try this in Python

import re

_USER = re.compile(r"^[a-z0-9_]{3,15}$")


def valid_username(s: str) -> bool:
    return bool(_USER.fullmatch(s))


print(valid_username("ab"), valid_username("ab_c12"))

Common mistakes

• Validating after mutation—capture user input snapshot first.
• Confusing validation (reject bad) with sanitization (fix silently)—product decision.

Key takeaways

• Return structured errors (missing_digit, too_short) instead of one generic false for better UX.
• Test boundary lengths 0, 1, max allowed.