What are some common security concerns or vulnerabilities associated with using regex patterns in PHP, and how can they be mitigated to protect against potential threats?

One common security concern when using regex patterns in PHP is the potential for injection attacks. To mitigate this risk, it is important to properly sanitize user input before using it in regex patterns. This can be done by using functions like `preg_quote()` to escape special characters.

$user_input = $_POST['user_input'];
$escaped_input = preg_quote($user_input, '/');
$pattern = '/^' . $escaped_input . '$/';