How can negative assertions be used to solve the issue mentioned in the forum thread?

Issue: The forum thread mentions a problem with user input validation where only positive assertions are being used, leading to potential security vulnerabilities. To solve this issue, negative assertions can be used to explicitly check for disallowed characters or patterns in the user input. PHP Code Snippet: 

// Check for disallowed characters using negative assertion
if (preg_match('/[^a-zA-Z0-9]/', $user_input)) {
    // Handle error for disallowed characters
    echo "Error: User input contains disallowed characters.";
} else {
    // Proceed with processing the user input
    echo "User input is valid.";
}

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