I think you want something like this,
> var s = '\n
\n ';
undefined
> var f = s.replace(/(href=")[^"]*([\s\S]*?)(src=")([^"]*)([\S\s]+?<\/a>)/g, '$1$4$2$3$4$5');
undefined
> var result = f.replace(/thumb(?=.*thumb)/, 'full');
undefined
> result
'\n
\n '