$re = '/(?:>|\s+)(?<word>@[a-zA-Z]\w+)(<|\s+)?/';
$str = '@word1 <p>ipsum dolor sit @word2 amet, consectetur adipiscing elit. Ut facilisis accumsan augue, <span>@word3</span>@word4 eget laoreet elit efficitur ut. sample@domain.com Fusce nulla ante, aliquam vel est sed, fringilla auctor risus. Ut consectetur mi non felis tincidunt, sit amet tincidunt massa placerat.</p>@word5';
preg_match_all($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