$re = '/(?:hdd\s+\dtb hdd \dtb(?!\s+hdd)|(?<!hdd\s)\dtb hdd \dtb\s+hdd)(*SKIP)(*F)|\dtb hdd \dtb/m';
$str = '{sometext...} 1tb hdd 1tb hdd {sometext...}
{sometext...} 1tb hdd 1tb {sometext...}
{sometext...} hdd 1tb hdd 1tb {sometext...}
{sometext...} hdd 1tb hdd 1tb hdd {sometext...}';
preg_match_all($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