$re = '/(?<!`)`{1,2}\b(?!`)(.*?)\b`+/';
$str = 'I `need` to match a ``word`` from a ```sentence`` Which `lies`` between `backquotes```` and this ``should```` also match and ```more``` than ```three```````` quotes ```````not``` matched';
preg_match_all($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