reverted:
--- b/core/modules/simpletest/lib/Drupal/simpletest/Tests/WebTestBaseTest.php
+++ /dev/null
@@ -1,60 +0,0 @@
-<?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 testAssertLink() {
-    $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 = '';
-    $this->plainTextContent = FALSE;
-    foreach ($link_texts as $link) {
-      $this->content .= '<a href="">' . $link .'</a>';
-    }
-
-    // Add some text that is not a link.
-    $non_link_text = $this->randomName(4);
-    $this->content .= '<p>' . $non_link_text . '</p>';
-
-    foreach ($link_texts as $link) {
-      $result = $this->assertLink($link);
-      $this->assertTrue($result, 'The assertLink method found link.');
-    }
-
-    // Check that the non link text exists but is not a link.
-    $this->assertText($non_link_text);
-    $this->assertNoLink($non_link_text);
-  }
-
-}
reverted:
--- b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -1880,14 +1880,7 @@
         // Return the string.
         $value = count($parts) > 1 ? 'concat(' . implode(', \'"\', ', $parts) . ')' : $parts[0];
       }
+      $xpath = preg_replace('/' . preg_quote($placeholder) . '\b/', $value, $xpath);
-
-      // Use preg_replace_callback so that if $value contains a string like "$1"
-      // this is not treated as a back reference.
-      $xpath = preg_replace_callback(
-        '/' . preg_quote($placeholder) . '\b/',
-        function ($matches) use ($value) { return $value; },
-        $xpath
-      );
     }
     return $xpath;
   }