$re = '/(?<year>\d{4})-(?<month>[01][0-9])-(?<day>[0123][0-9])/m';
$str = '2018-10-18
2018-1-1
2018-12-31
2018-01-01
';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php