Validate input length

Challenge

Implement int validate_user_input(const char *s) returning 0 if s is between 1 and 64 chars and contains only ASCII letters, digits, or one of -_.@+; otherwise -1.

Why this matters

The most common input bug isn't malicious content — it's malformed length. A function that asserts input length is in a known range catches 90% of bugs before they reach business logic.

Starter code

#include <stdio.h>
#include <ctype.h>

int validate_user_input(const char *s) {
    /* TODO */
    return -1;
}

Common mistakes

Off-by-one on the upper bound. Not checking for NULL. Using strlen on non-NUL-terminated input.

Edge cases to handle

Empty input. Input exactly at the limit. Input one byte over the limit. NULL pointer.

Complexity

O(length).

Background lessons

• Input validation
• String safety and bounded operations

Up next

• Write a safe allowlist validator
• Validate a password against a policy