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
@@ -865,7 +865,7 @@
   while ($max_end_index > $min_end_index) {
     // Check the index half way between min and max. See if we ended there,
     // if we would still have a match.
-    $proposed_end_index = floor(($max_end_index + $min_end_index) * 0.5);
+    $proposed_end_index = floor(($max_end_index + $min_end_index) / 2);
     $proposed_end_pos = $words[$proposed_end_index][1];
     // Since the split was done with preg_split(), the positions are byte counts
     // not character counts, so use substr() not Unicode::substr() here.
@@ -875,8 +875,8 @@
       $max_end_index = $proposed_end_index;
     }
     else {
-      // The proposed endpoint is too early, so the earliest possible
-      // OK ending would be the next word.
+      // The proposed endpoint index is too early, so the earliest possible
+      // OK ending point would be the next index.
       $min_end_index = $proposed_end_index + 1;
     }
   }
@@ -891,7 +891,7 @@
   while ($max_start_index > $min_start_index) {
     // Check the index half way between min and max. See if we started there,
     // if we would still have a match.
-    $proposed_start_index = ceil(($max_start_index + $min_start_index) * 0.5);
+    $proposed_start_index = ceil(($max_start_index + $min_start_index)  / 2);
     $proposed_start_pos = $words[$proposed_start_index][1];
     // Since the split was done with preg_split(), the positions are byte counts
     // not character counts, so use substr() not Unicode::substr() here.
@@ -901,14 +901,17 @@
       $min_start_index = $proposed_start_index;
     }
     else {
-      // The proposed start point is too late, so the latest possible
-      // OK starting point would be the previous word.
+      // The proposed start point index is too late, so the latest possible
+      // OK starting point would be the previous index.
       $max_start_index = $proposed_start_index - 1;
     }
   }
   $start_index = $max_start_index;
 
-  // Return the matching text.
+  // Return the matching text. We need to use substr() here and not the
+  // Unicode::substr() function, because the indices in $words came from
+  // preg_split(), so they are Unicode-safe byte positions, not character
+  // positions.
   return trim(substr($text, $words[$start_index][1], $words[$end_index][1] - $words[$start_index][1]));
 }