Javascript regexp：在反向引用模式中使用变量？

'url.com/foo/bar?this=that&thing=another'.replace(/(thing=)([^&]*)/, '$1test')

'url.com/foo/bar?this=that&thing=another'.replace('/(' + key + '=)([^&]*)/', '$1test')

GetSrcParam: function(key, value) {
            var newSrc = $(this._image).attr('src'),
                pattern = '(' + key + '=)([^&]*)';

            if (newSrc.match(pattern) == null)
                newSrc += '&' + key + '=' + value;
            else
                newSrc = newSrc.replace(newSrc, '$1' + value);

            return newSrc;
        }