$re = '/<style([\S\s]*?)>([\S\s]*?)<\/style>/miDJ';
$str = '<style>
.something1 {
display: flex;
color: red;
}
.something2 { display: flex; }
</style>
<div>cat</div>
<style>
.something3 {
display: flex;
color: red;
}
.something4 { display: flex; }
</style>';
preg_match_all($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