diff -u b/core/modules/search/search.module b/core/modules/search/search.module
--- b/core/modules/search/search.module
+++ b/core/modules/search/search.module
@@ -831,8 +831,8 @@
   $new_key = Unicode::strtolower($temp);
   $new_key = \Drupal::service('transliteration')->removeDiacritics($new_key);
   if (preg_match('/' . $boundary . preg_quote($new_key, '/') . $boundary . '/iu', ' ' . $new_text . ' ')) {
-    $position = strpos($new_text, $new_key);
-    return substr($text, $position, strlen($new_key));
+    $position = Unicode::strpos($new_text, $new_key);
+    return Unicode::substr($text, $position, Unicode::strlen($new_key));
   }
 
   // Run both text and key through search_simplify.
only in patch2:
unchanged:
--- a/core/lib/Drupal/Component/Utility/Unicode.php
+++ b/core/lib/Drupal/Component/Utility/Unicode.php
@@ -697,4 +697,29 @@ public static function validateUtf8($text) {
     return (preg_match('/^./us', $text) == 1);
   }
 
+  /**
+   * Finds the position of the first occurrence of a string in another string.
+   *
+   * @param string $haystack
+   *   The string to search in.
+   * @param string $needle
+   *   The string to find in $haystack.
+   * @param int $offset
+   *   If specified, start the search at this number of characters from the
+   *   beginning (default 0).
+   *
+   * @return int|false
+   *   The position where $needle occurs in $haystack, always relative to the
+   *   beginning (independent of $offset), or FALSE if not found. Note that
+   *   a return value of 0 is not the same as FALSE.
+   */
+  public static function strpos($haystack, $needle, $offset = 0) {
+    if (static::getStatus() == static::STATUS_MULTIBYTE) {
+      return mb_strpos($haystack, $needle, $offset);
+    }
+    else {
+      return strpos($haystack, $needle, $offset);
+    }
+  }
+
 }