adding an image class to all images to make lightbox work

I am using a lightbox plugin that requires class=“envirabox”. Unfortunately, the plugin doesn’t automatically add the class to existing image links like some of the other lightbox plugins do. So, I need to add the class to all images. I have a php function but it’s not working. In some cases, the images already have a class, so I need to add the envirabox class in. Maybe you guys can help me here.

function add_class($content) {
    global $post;
    $pattern        = "/(]*?class=['"].*)[^>]*?href=['"][^'"]+?.(?:bmp|gif|jpg|jpeg|png)['"][^>]*)>/i";
    $replacement    = '$1 class="envirabox">';
    $content = preg_replace($pattern, $replacement, $content);
    return $content;
}
add_filter("the_content","add_class");