I've written a small command to find all files which contain external url's, using
sublime $(ack -l '<a[^>]+href="http')
href="<do not contain website.nl>"
I believe ack uses Perl regex patterns, in which case you should use a negative look-ahead, like this
$ sublime $(ack -l '<a\b(?=[^>]+\bhref="http)(?![^>]+\brel="nofollow")')
But note that ack will check only one line at a time whereas an HTML
<a> element may run over several lines