$re = '/^Parameter #\d+ \[ <(?:optional|required)> ([a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*(?:\\\\[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*)*)(?: or NULL)? \$[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*(?: = NULL)? \]$/m';
$str = 'Parameter #1 [ <optional> Namespaced\\ClassName or NULL $className = NULL ]
Parameter #1 [ <required> Namespaced\\ClassName $className ]';
preg_match_all($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