Validate input length

Challenge

Accept a string only if every character is on an approved allowlist and its length is in range — the safe alternative to blocking known-bad characters.

Task

Implement int validate_user_input(const char *s) that returns 0 if s is valid and -1 otherwise.

s is valid when all of these hold:

• its length is between 1 and 64 characters (inclusive), and
• every character is an ASCII letter, an ASCII digit, or one of - _ . @ +.

Input

A NUL-terminated string s the grader passes. s may be NULL.

Output

Returns 0 if s is valid, -1 otherwise.

Example

validate_user_input("alice99@example.com")   ->   0
validate_user_input("alice.smith")           ->   0
validate_user_input("ali ce")                ->   -1   (space not allowed)
validate_user_input("alice;")                ->   -1   (';' not allowed)
validate_user_input("")                      ->   -1   (too short)
validate_user_input(NULL)                    ->   -1

Edge cases

• Empty string: invalid (length 0).
• Length exactly 64: valid; 65 or more: invalid.
• NULL pointer: return -1.

Rules

• Use an allowlist (approve known-good characters) rather than a blocklist.

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.

Input format

A NUL-terminated string s (may be NULL).

Output format

0 if s is 1..64 chars and only letters/digits/-_.@+; otherwise -1.

Constraints

Allowlist the permitted characters; reject everything else.

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