Be aware of bug https://bugs.php.net/bug.php?id=50887 when using sub patterns: Un-matched optional sub patterns at the end won't show up in $matches.

Here is a workaround: Assign a name to all subpatterns you are interested in, and merge $match afterwards with an constant array containing some reasonable default values:

<?php
if (preg_match('/^(?P<lang>[^;*][^;]*){1}(?:;q=(?P<qval>[0-9.]+))?$/u', 'de', $match))
{
  $match = array_merge(array('lang' => '', 'qval' => ''), $match);
  print_r($match);
}
?>

This outputs:
Array
(
    [lang] => de
    [qval] => 
    [0] => de
    [1] => de
)

Instead of:
Array
(
    [0] => de
    [lang] => de
    [1] => de
)