diff -u b/core/lib/Drupal/Core/Block/BlockBase.php b/core/lib/Drupal/Core/Block/BlockBase.php
--- b/core/lib/Drupal/Core/Block/BlockBase.php
+++ b/core/lib/Drupal/Core/Block/BlockBase.php
@@ -267,7 +267,7 @@
     $transliterated = $this->transliteration()->transliterate($admin_label, LanguageInterface::LANGCODE_DEFAULT, '_');
     $transliterated = Unicode::strtolower($transliterated);

-    $transliterated = preg_replace('@[^a-z0-9_.]+@', '', $transliterated);
+    $transliterated = preg_replace('@' . preg_quote($replace_pattern, '@') . '@', $replace, $transliterated);
 
     return $transliterated;
   }