When should regular expressions be used for input validation in PHP, and when are ctype functions more appropriate?

Regular expressions should be used for input validation in PHP when you need to match complex patterns or validate specific formats, such as email addresses or phone numbers. On the other hand, ctype functions are more appropriate for basic character checks, such as alphanumeric or numeric validation. It is important to choose the appropriate method based on the complexity of the validation required. 

// Regular expression for validating an email address
$email = "example@example.com";
if (preg_match("/^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/", $email)) {
    echo "Valid email address";
} else {
    echo "Invalid email address";
}

// Using ctype functions to check if a string contains only alphanumeric characters
$string = "abc123";
if (ctype_alnum($string)) {
    echo "String contains only alphanumeric characters";
} else {
    echo "String contains non-alphanumeric characters";
}

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