$re = '/class=("|"([^"]*)\s)share("|\s([^"]*)")/';
$str = '<div class="share"></div>
<div class="sdfsd share sdfsdfsdf"></div>
<a class="share-s">
<div class="share sdfsdfsdf"></div>
<a class="sharen">
<div class="sdfsd share"></div>
<a class="icon-share export-to-csv-button">
<a class="fxac link" href="/share ">
';
preg_match_all($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