$re = '/%(?P<flags>[-+ #0]*)(?P<width>\d+|\*)?(?:\.(?P<precision>\d+|\*))?(?P<length>hh|h|l|ll|j|z|t|L|I64)?(?P<specifier>[%csdioxXufFeEaAgGnp])/m';
$str = '%I64u';
preg_match_all($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