For reference links, modify the lines around line 733 (at the time of writing) to add nofollow
if appropriate. For inline links, modify line 752.
The reference link code currently looks like this:
$url = $this->urls[$link_id];
$url = $this->encodeAttribute($url);
$result = "<a href=\"$url\"";
if ( isset( $this->titles[$link_id] ) ) {
$title = $this->titles[$link_id];
$title = $this->encodeAttribute($title);
$result .= " title=\"$title\"";
}
$link_text = $this->runSpanGamut($link_text);
$result .= ">$link_text</a>";
$result = $this->hashPart($result);
The inline link code looks similar. You'll want to add a check for if $url
begins with !
and if so, strip it and put rel="nofollow"
at the right place in $result
.