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
@@ -1184,26 +1184,22 @@
    *   TRUE if the random string is valid, FALSE if not.
    */
   public function randomStringValidate($str) {
-    // Consecutive spaces causes issues for xpath
+    // Consecutive spaces causes issues for
+    // Drupal\simpletest\WebTestBase::assertLink().
     if (preg_match('/\s{2,}/', $str)) {
       return FALSE;
     }
 
-    // Starting with a space we can not guarantee length in UI.
+    // Starting with a space means that length might not be what is expected.
     if (preg_match('/^\s/', $str)) {
       return FALSE;
     }
 
-    // Ending with a space we can not guarantee length in UI.
+    // Ending with a space means that length might not be what is expected.
     if (preg_match('/\s$/', $str)) {
       return FALSE;
     }
 
-    // Containing \1 or $1 matches regex back references.
-    if (preg_match('/[\\\\|\$][0-9]/', $str)) {
-      return FALSE;
-    }
-
     return TRUE;
   }
 
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
@@ -10,7 +10,7 @@
 use Drupal\Tests\UnitTestCase;
 
 /**
- * Test PHPUnit errors are getting converted to Simpletest errors.
+ * Tests helper methods provided by the abstract TestBase class.
  */
 class TestBaseTest extends UnitTestCase {
 
@@ -24,7 +24,7 @@
   public static function getInfo() {
     return array(
       'name' => 'TestBase helper functions test',
-      'description' => 'Test helper functions provided by the TetsBase abstract class.',
+      'description' => 'Test helper functions provided by the TestBase abstract class.',
       'group' => 'Simpletest',
 
     );
@@ -34,6 +34,12 @@
     $this->stub = $this->getMockForAbstractClass('Drupal\simpletest\TestBase');
   }
 
+  /**
+   * Provides data for the random string validation test.
+   *
+   * @return array
+   *   An array of values passed to the test method.
+   */
   public function randomStringValidateProvider () {
     return array(
       array(' curry paste', FALSE),
@@ -42,17 +48,18 @@
       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)
     );
   }
 
   /**
+   * Tests the random strings validation rules.
+   *
    * @param string $str
+   *   The string to validate.
    * @param bool $expected
+   *   The expected result of the validation.
+   *
+   * @see \Drupal\simpletest\TestBase::randomStringValidate().
    *
    * @dataProvider randomStringValidateProvider
    */
diff -u b/core/tests/Drupal/Tests/Component/Utility/RandomTest.php b/core/tests/Drupal/Tests/Component/Utility/RandomTest.php
--- b/core/tests/Drupal/Tests/Component/Utility/RandomTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/RandomTest.php
@@ -152,7 +152,7 @@
   public function testRandomStringValidator() {
     $random = new Random();
     $this->firstStringGenerated = '';
-    $str = $random::string(1, FALSE, array($this, '_RandomStringValidate'));
+    $str = $random::string(1, TRUE, array($this, '_RandomStringValidate'));
     $this->assertNotEquals($this->firstStringGenerated, $str);
   }
 
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/simpletest/lib/Drupal/simpletest/Tests/WebTestBaseTest.php
@@ -0,0 +1,51 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\simpletest\Tests\WebTestBaseTest.
+ */
+
+namespace Drupal\simpletest\Tests;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests helper methods provided by the WebTestBase class.
+ */
+class WebTestBaseTest extends WebTestBase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'WebTestBase helper functions test',
+      'description' => 'Test helper functions provided by the WebTestBase class.',
+      'group' => 'Simpletest',
+    );
+  }
+
+  /**
+   * Tests the behaviour of Drupal\simpletest\WebTestBase::assertLink().
+   */
+  public function testAsertLink() {
+    $link_texts = array(
+      'curry paste',
+      'thai green curry paste',
+      'Number\\9',
+      'Number\9',
+      'Number9',
+      'Number9\\',
+      '$7@|{v$S',
+    );
+
+    // Set up some links to test.
+    $this->content = '';
+    foreach ($link_texts as $link) {
+      $this->content .= '<a href="">' . $link .'</a>';
+    }
+
+    foreach ($link_texts as $link) {
+      $result = $this->assertLink($link);
+      $this->assertTrue($result, 'The assertLink method found link.');
+    }
+  }
+
+}
only in patch2:
unchanged:
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -1877,7 +1877,7 @@ protected function buildXPathQuery($xpath, array $args = array()) {
         // Return the string.
         $value = count($parts) > 1 ? 'concat(' . implode(', \'"\', ', $parts) . ')' : $parts[0];
       }
-      $xpath = preg_replace('/' . preg_quote($placeholder) . '\b/', $value, $xpath);
+      $xpath = str_replace($placeholder, $value, $xpath);
     }
     return $xpath;
   }