Regex to filter captcha image src - php

Question

I'm trying to parse a curl response for some recaptcha image sadly my regex experience isn't that good, how could I find the src link in this code using regex?
<img id="recaptcha_challenge_image" alt="reCAPTCHA-Bild" height="57" width="300" src="https://www.google.com/recaptcha/api/image?c=03AHJ_VuvfHOCmjv_2G6NuZOIpbC-S7FGqqg-WM4ytRKoMD7oeXGHV_fvqim2YJSw3qkD7XcSxSlCOLAeBd4vy0S7KTrLiL7xSwc2YvGnc5Q9EUXllGa-56zXarU7Pq4btSzHn0anQPToXtBgDBdu69P-SwUpnzs1q08w9GTL8TEtpMAYYcW1hAXqpQEnPQO_h5tmtEDyr9WlgvVVTNcMwDsTJOnjsGmH4kg&th=,skNXQ2Kw6Nd0dqSGIn-2Ei0m-GKXNcjwAAAAcaAAAADlawORO7hNRBFlts00ZWNVrY7qTydBWIpzMqbUQsFjKKE1a8QhNJQqFgffS8OED15Kp3u0DRvaI7SCeuXwN6hkhGhMqCqgpe9cY2VyxbkFK8pRpnNcz5gnlz43NZsAyBZmvlX-BmLj6Hgq-6DzW0pWn3WFpGmM2UhRNudpy91Vyg7OwtD0xNhMGubBALOcAmR35Em3TsWYVh9mCL37V0nJ2xEBjjBQqCtn1Yoz5fLgxd3xp7229BR1zIremO1wJMCVljmT5uxYHyArQsN5PYqF-C7u4NkOSKS6PmkRTKAdV1NihEGAKxREfCsttZbVOY7MOVBF4zemvUDwIwA8pMJrnbbwwwXEinA2-w1NNS0tCxrubtqKCxu2EQsHRQ37V5NzI-Y_0qYMdxDa3wuXdS2ojVsjwXzVnBbYnG7LFJ1BJlZev4lugPtZh2siHolIbmHT8L1z3MMk0DEH0QnhNkd9x6e66tRyqYs7PsoteXmqa76sbqb945WtI5jsiJY4wo9yuKtGH03HmxdqhftgOk9OM6Gjjvhu1lxfW8tkOhehrGD5Td7z0L5fywtXmexRSlEQ5B4_OA3LEVmoCMUjW18GXDBj_lZPjAQ-mp6zV4a19ht88ilWfFTanLZ6d9FKsRrdwlNIS6cDzVBKT90mgXKARhibHrrSXujgo1l-gDbJ0o6xJBqSIugP155OVvwhJHW_ofOnvBuxgbvvsvOfskyGcFdnPoBIwrK-47AHx4H2jryUbCc3wLAtOcUicS_I2PRxKSUuUmYUk-bQq00scg0mDoI6QlD12pkPvmNA_QDyPqKjv5z8fc5HLVIAqFdBFdbWImHFKku0clxNX_qebl7r-C7e7LNBTngIFRdtFzAX_VjZHqRouemq2y89UA30WP65JSzzbUPt-z-tb6eKW3QD0eOlm28YkbYib9mdl85bIy61rS8bCHtFuKlcTMSzZyqMJhH25faKCTPkkXHhkPnO7IkMEmyll3LA5kjkc9RwTWFgF64RqLC-BqLscVi0GbVcCodMSVy1-kRGRqPr2ZaMwbLJSJq94Dy7reaex9rgiWEfpM0jEj_b1UeGUAEENhcPM3N63bPF3_F39H1YX3oBve4UXURVo7JkU2-C6o1HmB7Xr74JMEpPl8Vj1zImRk7SSB8Z6KEGv8Nj2f2Pq0hgaiehokt9I1JpnFprXtlEQW3vvgDZa01jYb6kmKVJNdaq0mIuvg">
My actual code:
<?php
class get_recaptcha {
function __construct(){
$this->website = 'http://registration.zwinky.com/registration/register.jhtml';
}
function curl_post(){
$ch = curl_init();
$timeout = 5;
curl_setopt($ch, CURLOPT_URL, $this->website);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, $timeout);
$data = curl_exec($ch);
curl_close($ch);
return $data;
}
function parse_content(){
preg_match('regexFilter', $this->curl_post(), $match);
$captcha_image = $match[1];
return $captcha_image;
}
}
$get_captcha = new get_recaptcha();
$captcha_response = $get_captcha->parse_content();
echo $captcha_response;
?>
Could anyone maybe help me setting up the regex filter for this?

Answer 1

It appears that the captcha is within an iframe so the url you would need to use the url for the iframe I think to get the captcha. Whilst not a RegEx solution this does get the desired information. If the url of the iframe changes then you'd need to muck about with the code to get the iframe src url - hope it helps.
$url='http://www.google.com/recaptcha/api/noscript?k=6Ldx8OgSAAAAAOQu76OwUC1XwCxpEZU576k0gHIR';
$html=file_get_contents( $url );
$dom=new DOMDocument;
$dom->loadHTML($html);
$col=$dom->getElementsByTagName('img');
foreach($col as $n)echo $n->getAttribute('src');
echo $src;