$re = '/"price":"([\d\.]+?)","opt":\{"[^a-z]*"4":\[(.+?)\]/';
$str = '"price":"7.99","opt":{"1":[1.01,1.02]},"mixedId":0,"price":"8.99","opt":{"3":[1.03],"4":[1.04,1.05]}';
preg_match($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