$re = '/^([0-9]{9}[0-9X]|[A-Za-z][\dA-Za-z]{9})\.([A-Z0-9]{4})\.PC_([0-9]{3})\.jpg$/';
$str = 'B12345678X.MAIN.PC_042.jpg';
preg_match($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