code: function() {
                      var value;
  
                      if (input.charAt(i) !== 'u' || !$(/^url\(/)) return;
                      value = $(this.entities.quoted) || $(this.entities.variable) ||
                              $(/^[\-\w%@$\/.&=:;#+?~]+/) || '';
                      if (! $(')')) {
                          return new tree.Invalid(value, memo, 'Missing closing ) in URL.', env.filename);
                      } else {
                          return new tree.URL((typeof value.value !== 'undefined' ||
                              value instanceof tree.Variable) ?
                              value : new tree.Quoted(value));
                      }
                  }
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.