$re = '/(?<=["\s])[^\s]+?\s?=\s?"\$\{(?:\w*)\}"/m';
$str = '<a onclick="javascript:collapse(\'displayText\',\'hideText\',\'whyInfo-arrow-image\')"><span sd:text="${whyInfoLabel}">Learn more about Authentication</span> <img id="whyInfo-arrow-image" sd:src="${downwardArrowImage}" height="20" width="20" >';
preg_match_all($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