Custom Drupal Filter to Remove HTTP Protocol

This is an example of a custom Drupal 7 input filter. This example will locate user content featuring images (etc) where the src attribute begins with http: and then remove the protocol portions. For example, <img src="http://example.com/image.jpg" alt=""> would become <img src="//example.com/image.jpg" alt="">, which allows the server to handle the request however it prefers. This came in handy for converting a large site over to https only, because it had a whole lot of embedded assets that would be impractical to update manually.

/**
 * Define custom input filters for our site.
 */
function MYMODULE_filter_info() {
  $filters = array();
  $filters['no-http'] = array(
    'title' => t('Strip http: protocol from src attributes. I should come after all other filters.'),
    'process callback' => '_MYMODULE_no_http_filter',
  );
  return $filters;
}

/**
 * This input filter strips http from any src attributes. We needed this because
 * the site owner has hotlinked and embedded content from all over the place
 * and tracking this all down might be an insurmountable challenge to the client.
 */
function _MYMODULE_no_http_filter($text, $filter, $format, $langcode, $cache, $cache_id) {
  return str_ireplace('src="http:', 'src="', $text);
}