Styling attachment id with css or preg_replace?

Dear all,

I’m building a photo website that scrolls horizontally where WP post will be image galleries.
I’d like my images to have captions but I can’t find a way to style with CSS the <div id="attachment_[number]...> I’m getting from the_content() (now it forces a line break… quite annoying for a site that’d like to scroll horizontally…).

The_content()’s output is as follows:

<div class="entry">
    <div id="attachment_123" class="wp-caption aligncenter">
        <a href="...">
           <img src="..."/>
        </a>
        <p class="wp-caption-text">Caption 1</p>
    </div>
    <div id="attachment_456" class="wp-caption aligncenter">
        <a href="...">
           <img src="..."/>
        </a>
        <p class="wp-caption-text">Caption 2</p>
    </div>
</div>

Is there a simple way to style those attachment_[number] id with a changing [number] or do I have to modify the output of the_content() with a preg_replace? How then?

Regards,

Matteo