$re = '/^[^<>{}\n]*(?:(?:<[^<>\n]*>|{[^{}\n]*})+[^<>{}\n]*)*$/m';
$str = 'XXX<XX{X}XXX>X
X{X}X<X>X{X}X<X>X
XXX<X<XX>>XX
XX<XX{XX>XX}XX';
preg_match_all($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