$re = '/^(?:((бл\.\s?)?\p{N}{1,4}[\s-]?\p{Cyrillic}?)|((\p{N}{1,4}-){0,2}\p{N}{1,4}))$/A';
$str = '2-4-6-8';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($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