php关键词标红的算法

下面是PHP代码：

function highlight_keywords($text, $keyword) {
$keyword = preg_quote($keyword, ‘/’);
$pattern = “/(?!<[^<>])(?<![?.&])\b($keyword)\b(?![^<>]*>)/i”;
return preg_replace($pattern, ‘<span style=”color:red;”>$1</span>’, $text);
}