RegEx expression to find a href links and add NoFollow to them

Question

I am trying to write a RegEx rule to find all a href HTML links on my webpage and add a \'rel=\"nofollow\"\' to them.

However, I have a list of URLs that must be exc

Answer 1

An improvement to James' regex:

(]*)(href="https?://)((?!(?:(?:www\.)?'.implode('|(?:www\.)?', $follow_list).'))[^"]+)"((?!.*\brel=)[^>]*)(?:[^>]*)>

This regex will matches links NOT in the string array $follow_list. The strings don't need a leading 'www'. :) The advantage is that this regex will preserve other arguments in the tag (like target, style, title...). If a rel argument already exists in the tag, the regex will NOT match, so you can force follows on urls not in $follow_list

Replace the with:

$1$2$3"$4 rel="nofollow">

Full example (PHP):

function dont_follow_links( $html ) {
 // follow these websites only!
 $follow_list = array(
  'google.com',
  'mypage.com',
  'otherpage.com',
 );
 return preg_replace(
  '%(]*)(href="https?://)((?!(?:(?:www\.)?'.implode('|(?:www\.)?', $follow_list).'))[^"]+)"((?!.*\brel=)[^>]*)(?:[^>]*)>%',
  '$1$2$3"$4 rel="nofollow">',
  $html);
}

If you want to overwrite rel no matter what, I would use a preg_replace_callback approach where in the callback the rel attribute is replaced separately:

$subject = preg_replace_callback('%(]*href="https?://(?:(?!(?:(?:www\.)?'.implode('|(?:www\.)?', $follow_list).'))[^"]+)"[^>]*)>%', function($m) {
    return preg_replace('%\srel\s*=\s*(["\'])(?:(?!\1).)*\1(\s|$)%', ' ', $m[1]).' rel="nofollow">';
}, $subject);