diff -u b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
--- b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -1199,8 +1199,8 @@
       return FALSE;
     }
 
-    // Containing \1 matches regex back references.
-    if (preg_match('/\\\\[0-9]/', $str)) {
+    // Containing \1 or $1 matches regex back references.
+    if (preg_match('/[\\\\|\$][0-9]/', $str)) {
       return FALSE;
     }
 
diff -u b/core/modules/simpletest/tests/Drupal/simpletest/Tests/TestBaseTest.php b/core/modules/simpletest/tests/Drupal/simpletest/Tests/TestBaseTest.php
--- b/core/modules/simpletest/tests/Drupal/simpletest/Tests/TestBaseTest.php
+++ b/core/modules/simpletest/tests/Drupal/simpletest/Tests/TestBaseTest.php
@@ -36,16 +36,17 @@
 
   public function randomStringValidateProvider () {
     return array(
-      array(' rewrew', FALSE),
-      array('rewrew ', FALSE),
-      array('re  wrew', FALSE),
-      array('re   wrew', FALSE),
-      array('re wrew', TRUE),
-      array('re wr ew', TRUE),
-      array('sdsfs\\5', FALSE),
-      array('sdsfs\5', FALSE),
-      array('sdsfs5', TRUE),
-      array('sds\\fs5', TRUE),
+      array(' curry paste', FALSE),
+      array('curry paste ', FALSE),
+      array('curry  paste', FALSE),
+      array('curry   paste', FALSE),
+      array('curry paste', TRUE),
+      array('thai green curry paste', TRUE),
+      array('Number\\9', FALSE),
+      array('Number\9', FALSE),
+      array('Number9', TRUE),
+      array('Number9\\', TRUE),
+      array('$7@|{v$S', FALSE)
     );
   }