[Silverstripe] add <a href="big.png"> to all <img class"popup" src="small.png"> to allow opening full image

Page.php

class Page extends SiteTree {

...

   function Content() {
      $content = $this->Content;
      // move class to the beginning right after <img
      $pattern[0] = '/(<img)([^>]*?)(class=(\'|")([^>]*?)(\'|"))([^>]*?>)/i';
      $replacement[0] = '$1 $3$2$7';
      // add <a> to all images with class popup and a src having _resampled
      $pattern[1] = '/(<img[^>]*?class=("|\')[^>]*?popup[^>]*?("|\')[^>]*?src=("|\')([^>]*?)_resampled\/[^\->]*?-([^>]*?)("|\')[^>]*?>)/i';
      $replacement[1] = '<a href="$4$6">$1</a>';
      return preg_replace($pattern, $replacement, $content);
   }

...

NOTE: one might want to test it before using it (: