How can I add custom link validation?

Question:

Is there a possibility to configure links in the editor to only allow relative links (e.g. /index.php) or links from specific domains (e.g. https://www.example.com/index.php)?

Answer:

You could use the link.beforeInsert event that was added in V2.4.0. Here is a snippet that allows to insert only absolute links (those that start with /) inside the Froala WYSIWYG HTML Editor:

<script>
    $(function(){
      $('#edit').froalaEditor({
          linkAutoPrefix: ''
        })
        .on('froalaEditor.link.beforeInsert', function (e, editor, link, text, attrs) {
          if (link.indexOf('/') !== 0) {
            alert ('Only absolute links are allowed!');
            return false;
          }
        })
    });
</script>

Do not forget to disable linkAutoPrefix option as well by setting it to an empty string.