$re = '/(?<=5;)(?P<date>\d{8})|(?<=;3;)(?P<value>\d+(?:\.\d+)?)/m';
$str = ';2;4;2;5;20181201;3;102;5;20191201;3;101.000000;5;20201201;3;100.000000;5;20211201;3;100.000000;';
preg_match_all($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