GUI of regex_redirect does not support targets without a pattern

I am working on this.

This is ready for review. The validation of the entered target path is not done by the setRegexErrors function; which feels deliberate.

Seems to be working as expected. I'm not sure if a setting for this functionality is actually needed @LaravZ.

@ajits: You did leave an unused variable in there, though

diff --git a/src/Form/RegexRedirectForm.php b/src/Form/RegexRedirectForm.php
index a39041c..fdeb385 100644
--- a/src/Form/RegexRedirectForm.php
+++ b/src/Form/RegexRedirectForm.php
@@ -217,7 +217,6 @@ class RegexRedirectForm extends ContentEntityForm {
     // Set the regex delimiter in code to avoid issues with leading slash.
     $delimiter = '/';
     $source_regex = $delimiter . trim($source_path) . $delimiter;
-    $redirect_regex = str_replace('internal:', '', $redirect_uri);
 
     if (preg_match($source_regex, '') === FALSE) {
       $form_state->setErrorByName('regex_redirect_source', $this->t('The regex redirect source must be a regex pattern.'));

Removed unused variable.

Looks good now.
Thanks @ajits.

Marking RTBC.