$re = '/(?<![\w)]\s\s|[\w)]\s|[\w)])((\{)(.*?)\})/ism';
$str = '
@media(max-width: 450px){
.est{
direction: ltr;}
}
{
color: red
}
a
{
color: red
}
{
color: red
}
{
color: red
}
a
{
color: red
}
a
{}
{}
';
preg_match_all($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