$re = '/ \w{4,6}:nil="true" xmlns.*\/>/';
$str = '
<SonOlcumIstasyon d3p1:nil="true" xmlns:d3p1="http://www.w3.org/2001/XMLSchema-instance" />
';
preg_match_all($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