Attributes = function(element, attr) {
      this.$$element = element;
      this.$attr = attr || {};
    }
@ngdoc function
@name ng.$compileProvider#imgSrcSanitizationWhitelist
@methodOf ng.$compileProvider
@function

@description
Retrieves or overrides the default regular expression that is used for whitelisting of safe
urls during img[src] sanitization.

The sanitization is a security measure aimed at prevent XSS attacks via html links.

Any url about to be assigned to img[src] via data-binding is first normalized and turned into
an absolute url. Afterwards, the url is matched against the `imgSrcSanitizationWhitelist`
regular expression. If a match is found, the original url is written into the dom. Otherwise,
the absolute url is prefixed with `'unsafe:'` string and only then is it written into the DOM.

@param {RegExp=} regexp New regexp to whitelist urls with.
@returns {RegExp|ng.$compileProvider} Current RegExp if called without value or self for
   chaining otherwise.