code: function () {
                      var value;
  
                      if (input.charAt(i) !== 'u' || !$re(/^url\(/)) {
                          return;
                      }
  
                      value = this.quoted() || this.variable() ||
                              $re(/^(?:(?:\\[\(\)'"])|[^\(\)'"])+/) || "";
  
                      expectChar(')');
  
                      return new(tree.URL)((value.value != null || value instanceof tree.Variable)
                                          ? value : new(tree.Anonymous)(value), env.currentFileInfo);
                  }
docs: Parse url() tokens  We use a specific rule for urls, because they don't really behave like standard function calls. The difference is that the argument doesn't have to be enclosed within a string, so it can't be parsed as an Expression.