使用带有wordpress的preg_replace（）过滤the_content

Question

I am trying to append a php query to the end of links from my website to a sponsor site using the_content filter and preg_replace(). I have tested my regex expression on regexr.com and it works. I have also used print_r() to test that the function is being called but for some reason the links are not being changed in practice. Here is the code I am having trouble with

add_filter('the_content', 'linkAppend');

function linkAppend($content) {
  global $referalString;

  preg_replace('/\/\/(www|launch)?\.?(solarwinds\.com)\/[^"]*/g','$&?cmp='.$referalString, $content);
  return $content;
}   

If someone could point me in the right direction, or let me know where I went wrong, I would be greatly appreciate it.

Answer 1

Turns out I had multiple problems, it was pointed out to me that I wasn't assigning the output of my preg_replace( ) to $content and then I found and solved issues with the global flag on the regular expression not being valid in php and me not accounting for links without a / after the .com. Final fix looks like this:

add_filter('the_content', 'linkAppend');

function linkAppend($content) {
  global $referalString;
  $content = preg_replace('/\/\/(www|launch)?\.?(solarwinds\.com)\/?[^"]*/m','$0?cmp='.$referalString, $content);
  return $content;
}