Regular Expression to replace link in case having no particular class - regex

Question

I tried real hard to find solution but couldn't do. Yup regex is way too complex. Anyways here is problem.
Objective:
I want to replace image link with cdn image links in PHP. In order to do that I thought better is to use preg_replace.
if links is /var/b.png OR http://www.example.com/png it will be replaced with CDN but if case src or class contains 'captcha' then it shouldn't as these are dynamic in nature.
For start I am trying:
$_SERVER["HTTP_HOST"] = 'www.bring.com';
$preg_host = preg_quote($_SERVER["HTTP_HOST"], '/');
$content = preg_replace('/((\<image\s+.*?src\=)(["\']http\:\/\/'.$preg_host.')(\/.*?["\'](^(?=.*(captcha)))(.*)?\>))/i', '$2$3.nyud.net:8080$4', $content);
$content = preg_replace('/(\<image\s+.*?src\=["\'])(\/.*?["\'].*?\>)/i', '$1http://'.$_SERVER['HTTP_HOST'].'.nyud.net:8080$2', $content);
Condition is that:
When not to do: src can contain "captcha" word and in some cases class contains "captcha" and this class can ahead or src or behind src which is making it more complicated. In these cases I don't want to replace links for example:
$content = <<<END
<image
type="image" src="/skins/bph/customer/images/icons/go.gif" alt="Search" title="Search" class="go-button" />
<image
id="verification_image_login_login_popup_form" src="http://www.bring.com/index.php?dispatch=image.captcha&verification_id=%3Alogin_login_popup_form&login_login_popup_form4ef33269bf30b=" alt="" onclick="this.src += 'reload' ;" width="100" height="25" class="image-captcha valign" /></p><div
class="clear">
<image
id="verification_image_login_login_popup_form" class="valign" src="http://www.bring.com/skins/bph/customer/images/icons/go.gif" alt="" onclick="this.src += 'reload' ;" width="100" height="25" /></p><div
class="clear">
END;
So as a result:
Shouldn't be replaced, but is happening opposite :(
Following should get replace as it doesn't have any class with captcha or link with captcha word in it
<image
id="verification_image_login_login_popup_form" class="valign" src="http://www.bring.com/skins/bph/customer/images/icons/xxx" alt="" onclick="this.src += 'reload' ;" width="100" height="25" /></p>

Answer 1

Rather than trying to solve whole problem by using regex magic (which can bite you at unexpected times) it is highly recommended to use PHP DOM parser.
Using DOM parser iterate through all the images and examine their src and class attributes and make your link modification as needed.
You can see tons of examples on using DOM if you search it here on SO or on Google.