diff -u b/core/lib/Drupal/Component/Utility/SafeMarkup.php b/core/lib/Drupal/Component/Utility/SafeMarkup.php
--- b/core/lib/Drupal/Component/Utility/SafeMarkup.php
+++ b/core/lib/Drupal/Component/Utility/SafeMarkup.php
@@ -153,14 +153,23 @@
   }
 
   /**
-   * @todo Document this function.
+   * Filters field separators.
+   *
+   * Strings smaller than 3 characters will not be set as safe.
    *
    * @param string $string
+   *   A string.
    *
-   * @return SafeMarkup|string
+   * @return string
+   *   The escaped string, which is marked as safe if greater than 2 characters.
    */
-  public static function filterSeparator($string) {
-    return twig_drupal_join_filter(array($string));
+  public static function filterSeparator($string) {;
+    if (strlen($string) < 3) {
+      return '';
+    }
+    $output = SafeMarkup::escape($string);
+    return SafeMarkup::set($output);
+
   }
 
   /**
only in patch2:
unchanged:
--- a/core/tests/Drupal/Tests/Component/Utility/SafeMarkupTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/SafeMarkupTest.php
@@ -101,4 +101,20 @@ public function testInvalidSetMultiple() {
     SafeMarkup::setMultiple($texts);
   }
 
+  /**
+   * Tests SafeMarkup::filterSeparator().
+   *
+   * @covers ::filterSeparator
+   */
+  public function testFilterSeparator() {
+    $returned = SafeMarkup::filterSeparator('<br />');
+    $this->assertEquals('&lt;br /&gt;', $returned, 'Separator string "<br />" was escaped.');
+    $this->assertTrue(SafeMarkup::isSafe($returned), 'Separator string "<br />" was marked as safe');
+    $returned = SafeMarkup::filterSeparator('---');
+    $this->assertEquals('---', $returned, 'Separator string "---" was not modified by the filter');
+    $this->assertTrue(SafeMarkup::isSafe($returned), 'Separator string "---" was marked as safe');
+    $returned = SafeMarkup::filterSeparator('<');
+    $this->assertEquals('', $returned, 'Separator string "<" is unsafe and was stripped');
+    $this->assertFalse(SafeMarkup::isSafe($returned), 'Separator string "<" was not marked as safe');
+  }
 }