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
@@ -646,7 +646,9 @@
  */
 function search_excerpt($keys, $text, $langcode = NULL) {
   // We highlight around non-indexable or CJK characters.
-  $boundary = '(?:(?<=[' . Unicode::PREG_CLASS_WORD_BOUNDARY . PREG_CLASS_CJK . '])|(?=[' . Unicode::PREG_CLASS_WORD_BOUNDARY . PREG_CLASS_CJK . ']))';
+  $boundary_character = '[' . Unicode::PREG_CLASS_WORD_BOUNDARY . PREG_CLASS_CJK . ']';
+  $preceded_by_boundary = '(?<=' . $boundary_character . ')';
+  $followed_by_boundary = '(?=' . $boundary_character . ')';
 
   // Extract positive keywords and phrases.
   preg_match_all('/ ("([^"]+)"|(?!OR)([^" ]+))/', ' ' . $keys, $matches);
@@ -662,7 +664,7 @@
   // search_simplify().
   $temp_keys = array();
   foreach ($keys as $key) {
-    $key = _search_find_match_with_simplify($key, $text, $boundary, $langcode);
+    $key = _search_find_match_with_simplify($key, $text, $boundary_character, $langcode);
     if (isset($key)) {
       // Quote slashes so they can be used in regular expressions.
       $temp_keys[] = preg_quote($key, '/');
@@ -697,7 +699,7 @@
       // we are requiring a match on a word boundary, make sure $text starts
       // and ends with a space.
       $matches = array();
-      if (preg_match('/' . $boundary . $key . $boundary . '/iu', ' ' . $text . ' ', $matches, PREG_OFFSET_CAPTURE, $look_start[$key])) {
+      if (preg_match('/' . $preceded_by_boundary . $key . $followed_by_boundary . '/iu', ' ' . $text . ' ', $matches, PREG_OFFSET_CAPTURE, $look_start[$key])) {
         $found_position = $matches[0][1];
         $look_start[$key] = $found_position + 1;
         // Keep track of which keys we found this time, in case we need to
@@ -783,7 +785,7 @@
 
   // Highlight keywords. Must be done at once to prevent conflicts ('strong'
   // and '<strong>').
-  $text = trim(preg_replace('/' . $boundary . '(?:' . implode('|', $keys) . ')' . $boundary . '/iu', '<strong>\0</strong>', ' ' . $text . ' '));
+  $text = trim(preg_replace('/' . $preceded_by_boundary . '(?:' . implode('|', $keys) . ')' . $followed_by_boundary . '/iu', '<strong>\0</strong>', ' ' . $text . ' '));
   return [
     '#markup' => $text,
     '#allowed_tags' => ['strong']
@@ -802,7 +804,8 @@
  * @param string $text
  *   The text to search for the keyword.
  * @param string $boundary
- *   Regular expression for boundary characters between words.
+ *   Regular expression for the boundary character class (characters that
+ *   indicate spaces between words).
  * @param string|null $langcode
  *   Language code for the language of $text, if known.
  *
@@ -813,6 +816,9 @@
  *   not located, NULL is returned.
  */
 function _search_find_match_with_simplify($key, $text, $boundary, $langcode = NULL) {
+  $preceded_by_boundary = '(?<=' . $boundary . ')';
+  $followed_by_boundary = '(?=' . $boundary . ')';
+
   // See if $key appears as-is. When testing, make sure $text starts/ends with
   // a space, because we require $key to be surrounded by word boundary
   // characters.
@@ -820,7 +826,7 @@
   if ($temp == '') {
     return NULL;
   }
-  if (preg_match('/' . $boundary . preg_quote($temp, '/') . $boundary . '/iu', ' ' . $text . ' ')) {
+  if (preg_match('/' . $preceded_by_boundary . preg_quote($temp, '/') . $followed_by_boundary . '/iu', ' ' . $text . ' ')) {
     return $temp;
   }
 
@@ -830,7 +836,7 @@
   $new_text = \Drupal::service('transliteration')->removeDiacritics($new_text);
   $new_key = Unicode::strtolower($temp);
   $new_key = \Drupal::service('transliteration')->removeDiacritics($new_key);
-  if (preg_match('/' . $boundary . preg_quote($new_key, '/') . $boundary . '/iu', ' ' . $new_text . ' ')) {
+  if (preg_match('/' . $preceded_by_boundary . preg_quote($new_key, '/') . $followed_by_boundary . '/u', ' ' . $new_text . ' ')) {
     $position = Unicode::strpos($new_text, $new_key);
     return Unicode::substr($text, $position, Unicode::strlen($new_key));
   }
@@ -844,7 +850,7 @@
   }
 
   // Split $text into words, keeping track of where the word boundaries are.
-  $words = preg_split('/' . $boundary . '/iu', $text, NULL, PREG_SPLIT_OFFSET_CAPTURE);
+  $words = preg_split('/' . $boundary . '+/u', $text, NULL, PREG_SPLIT_OFFSET_CAPTURE);
   // Add an entry pointing to the end of the string, for the loop below.
   $words[] = array('', strlen($text));
   $num_words = count($words);