- filter_var: documentation ( source)
<?php
echo "Output: " . filter_var('!#$%&\'*+-=?^_`{|}~@.[]', FILTER_SANITIZE_EMAIL);
echo "\nOutput: " . filter_var('!#$%&\'*+-=?^_`{|}~@.[]/', FILTER_SANITIZE_EMAIL);
echo "\nOutput: " . filter_var('/', FILTER_SANITIZE_EMAIL);