Code:
$s = preg_replace("/\[video=\bhttps?:\/\/www\.youtube\.com\/watch\?v=([\w\d]+?)\]/i","<iframe width=" . "\"420\" " . "height=\"315\"" . "src=\"http://www.youtube.com/embed/" . "\\1\"" . "frameborder=\"0\"" . "allowfullscreen>" . "</iframe>", $s);
Usage
[video=https://www.youtube.com/watch?v=IfR_DOb1TLI]