src/vs/css.build.ts/CSSPluginUtilities/replaceURL
class CSSPluginUtilities:	public static replaceURL(contents: string, replacer: (url: string) => string): string {
		// Use ")" as the terminator as quotes are oftentimes not used at all
		return contents.replace(/url\(\s*([^\)]+)\s*\)?/g, (_: string, ...matches: string[]) => {
			let url = matches[0];
			// Eliminate starting quotes (the initial whitespace is not captured)
			if (url.charAt(0) === '"' || url.charAt(0) === '\'') {
				url = url.substring(1);
			}
			// The ending whitespace is captured
			while (url.length > 0 && (url.charAt(url.length - 1) === ' ' || url.charAt(url.length - 1) === '\t')) {
				url = url.substring(0, url.length - 1);
			}
			// Eliminate ending quotes
			if (url.charAt(url.length - 1) === '"' || url.charAt(url.length - 1) === '\'') {
				url = url.substring(0, url.length - 1);
			}

			if (!CSSPluginUtilities.startsWith(url, 'data:') && !CSSPluginUtilities.startsWith(url, 'http://') && !CSSPluginUtilities.startsWith(url, 'https://')) {
				url = replacer(url);
			}

			return 'url(' + url + ')';
		});
	}