code: function validateCubemapSrc (src, cb) {
    var aCubemap;
    var cubemapSrcRegex = '';
    var i;
    var urls;
    var validatedUrls = [];
  
    for (i = 0; i < 5; i++) {
      cubemapSrcRegex += '(url\\((?:[^\\)]+)\\),\\s*)';
    }
    cubemapSrcRegex += '(url\\((?:[^\\)]+)\\)\\s*)';
    urls = src.match(new RegExp(cubemapSrcRegex));
  
    // `src` is a comma-separated list of URLs.
    // In this case, re-use validateSrc for each side of the cube.
    function isImageCb (url) {
      validatedUrls.push(url);
      if (validatedUrls.length === 6) {
        cb(validatedUrls);
      }
    }
    if (urls) {
      for (i = 1; i < 7; i++) {
        validateSrc(parseUrl(urls[i]), isImageCb);
      }
      return;
    }
  
    // `src` is a query selector to <a-cubemap> containing six $([src])s.
    aCubemap = validateAndGetQuerySelector(src);
    if (!aCubemap) { return; }
    if (aCubemap.tagName === 'A-CUBEMAP' && aCubemap.srcs) {
      return cb(aCubemap.srcs);
    }
    // Else if aCubeMap is not a <a-cubemap>.
    warn('Selector "%s" does not point to <a-cubemap>', src);
  }
docs: Validates six images as a cubemap, either as selector or comma-separated
  URLs.
  
  @param {string} src - A selector or comma-separated image URLs. Image URLs
  must be wrapped by `url()`.
  @param {string} src - A selector or comma-separated image URLs. Image URLs
  must be wrapped by `url()`.