diff --git a/core/includes/file.inc b/core/includes/file.inc
index 545a4a2..d241e15 100644
--- a/core/includes/file.inc
+++ b/core/includes/file.inc
@@ -914,7 +914,7 @@ function file_unmanaged_save_data($data, $destination = NULL, $replace = FILE_EX
  * @param $options
  *   An associative array of additional options, with the following elements:
  *   - 'nomask': The preg_match() regular expression for files to be excluded.
- *     There is no default.
+ *     Defaults to the 'file_scan_ignore_directories' setting.
  *   - 'callback': The callback function to call for each match. There is no
  *     default callback.
  *   - 'recurse': When TRUE, the directory scan will recurse the entire tree
@@ -953,6 +953,9 @@ function file_scan_directory($dir, $mask, $options = array(), $depth = 0) {
   // performance boost.
   if (!isset($options['nomask'])) {
     $ignore_directories = Settings::get('file_scan_ignore_directories', []);
+    array_walk($ignore_directories, function(&$value) {
+      $value = preg_quote($value, '/');
+    });
     $default_nomask = '/^' . implode('|', $ignore_directories) . '$/';
   }
 
diff --git a/core/modules/system/src/Tests/File/ScanDirectoryTest.php b/core/modules/system/src/Tests/File/ScanDirectoryTest.php
index 96fdc33..9c85d16 100644
--- a/core/modules/system/src/Tests/File/ScanDirectoryTest.php
+++ b/core/modules/system/src/Tests/File/ScanDirectoryTest.php
@@ -7,8 +7,6 @@
 
 namespace Drupal\system\Tests\File;
 
-use Drupal\Core\Site\Settings;
-
 /**
  * Tests the file_scan_directory() function.
  *
@@ -160,6 +158,12 @@ function testIgnoreDirectories() {
     $files = file_scan_directory('core/modules/system/tests/fixtures/IgnoreDirectories', '/\.txt$/');
     $this->assertEqual(1, count($files), '1 text files found when ignoring directories called "frontend_framework".');
 
+    // Ensure that the directories in file_scan_ignore_directories are escaped
+    // using preg_quote.
+    $this->settingsSet('file_scan_ignore_directories', ['frontend.*']);
+    $files = file_scan_directory('core/modules/system/tests/fixtures/IgnoreDirectories', '/\.txt$/');
+    $this->assertEqual(2, count($files), '2 text files found when ignoring a directory that is not there.');
+
     $files = file_scan_directory('core/modules/system/tests/fixtures/IgnoreDirectories', '/\.txt$/', ['nomask' => '/^something_thing_else$/']);
     $this->assertEqual(2, count($files), '2 text files found when an "nomasl" option passed in.');
   }
diff --git a/sites/default/default.settings.php b/sites/default/default.settings.php
index 1df5da2..c99aba9 100644
--- a/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -705,10 +705,14 @@
  */
 
 /**
- * The list of folders disabled for directory scanning.
+ * The default list of directories that will be ignored by Drupal's file API's.
  *
  * By default ignore node_modules and bower_components folders to avoid issues
- * with Drupal recursive scanning.
+ * with common frontend tools and recursive scanning of directories looking for
+ * extensions.
+ *
+ * @see file_scan_directory()
+ * @see \Drupal\Core\Extension\ExtensionDiscovery::scanDirectory()
  */
 $settings['file_scan_ignore_directories'] = [
   'node_modules',