karmen/node_modules/style-loader/lib/urls.js

/**
 * When source maps are enabled, `style-loader` uses a link element with a data-uri to
 * embed the css on the page. This breaks all relative urls because now they are relative to a
 * bundle instead of the current page.
 *
 * One solution is to only use full urls, but that may be impossible.
 *
 * Instead, this function "fixes" the relative urls to be absolute according to the current page location.
 *
 * A rudimentary test suite is located at `test/fixUrls.js` and can be run via the `npm test` command.
 *
 */

module.exports = function (css) {
  // get current location
  var location = typeof window !== "undefined" && window.location;

  if (!location) {
    throw new Error("fixUrls requires window.location");
  }

	// blank or null?
	if (!css || typeof css !== "string") {
	  return css;
  }

  var baseUrl = location.protocol + "//" + location.host;
  var currentDir = baseUrl + location.pathname.replace(/\/[^\/]*$/, "/");

	// convert each url(...)
	/*
	This regular expression is just a way to recursively match brackets within
	a string.

	 /url\s*\(  = Match on the word "url" with any whitespace after it and then a parens
	   (  = Start a capturing group
	     (?:  = Start a non-capturing group
	         [^)(]  = Match anything that isn't a parentheses
	         |  = OR
	         \(  = Match a start parentheses
	             (?:  = Start another non-capturing groups
	                 [^)(]+  = Match anything that isn't a parentheses
	                 |  = OR
	                 \(  = Match a start parentheses
	                     [^)(]*  = Match anything that isn't a parentheses
	                 \)  = Match a end parentheses
	             )  = End Group
              *\) = Match anything and then a close parens
          )  = Close non-capturing group
          *  = Match anything
       )  = Close capturing group
	 \)  = Match a close parens

	 /gi  = Get all matches, not the first.  Be case insensitive.
	 */
	var fixedCss = css.replace(/url\s*\(((?:[^)(]|\((?:[^)(]+|\([^)(]*\))*\))*)\)/gi, function(fullMatch, origUrl) {
		// strip quotes (if they exist)
		var unquotedOrigUrl = origUrl
			.trim()
			.replace(/^"(.*)"$/, function(o, $1){ return $1; })
			.replace(/^'(.*)'$/, function(o, $1){ return $1; });

		// already a full url? no change
		if (/^(#|data:|http:\/\/|https:\/\/|file:\/\/\/)/i.test(unquotedOrigUrl)) {
		  return fullMatch;
		}

		// convert the url to a full url
		var newUrl;

		if (unquotedOrigUrl.indexOf("//") === 0) {
		  	//TODO: should we add protocol?
			newUrl = unquotedOrigUrl;
		} else if (unquotedOrigUrl.indexOf("/") === 0) {
			// path should be relative to the base url
			newUrl = baseUrl + unquotedOrigUrl; // already starts with '/'
		} else {
			// path should be relative to current directory
			newUrl = currentDir + unquotedOrigUrl.replace(/^\.\//, ""); // Strip leading './'
		}

		// send back the fixed url(...)
		return "url(" + JSON.stringify(newUrl) + ")";
	});

	// send back the fixed css
	return fixedCss;
};