diff --git a/core/modules/contact/src/Tests/ContactPersonalTest.php b/core/modules/contact/src/Tests/ContactPersonalTest.php
index 51124c9..d941296 100644
--- a/core/modules/contact/src/Tests/ContactPersonalTest.php
+++ b/core/modules/contact/src/Tests/ContactPersonalTest.php
@@ -92,7 +92,7 @@ function testSendPersonalContactMessage() {
       '@sender_email' => $this->web_user->getEmail(),
       '@recipient_name' => $this->contact_user->getUsername()
     );
-    $this->assertText(String::format('@sender_name (@sender_email) sent @recipient_name an email.', $placeholders));
+    $this->assertRaw(String::format('@sender_name (@sender_email) sent @recipient_name an email.', $placeholders));
   }
 
   /**
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 98f68fc..68d5873 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -527,7 +527,7 @@ protected function drupalCreateUser(array $permissions = array(), $name = NULL)
 
     // Create a user assigned to that role.
     $edit = array();
-    $edit['name']   = !empty($name) ? $name : $this->randomMachineName();
+    $edit['name']   = !empty($name) ? $name : $this->randomMachineName() . '+' . $this->randomMachineName();
     $edit['mail']   = $edit['name'] . '@example.com';
     $edit['pass']   = user_password();
     $edit['status'] = 1;
diff --git a/core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php b/core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php
index 5b788ca..7e9ed63 100644
--- a/core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php
+++ b/core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php
@@ -33,7 +33,7 @@ public function validate($items, Constraint $constraint) {
     if (strpos($name, '  ') !== FALSE) {
       $this->context->addViolation($constraint->multipleSpacesMessage);
     }
-    if (preg_match('/[^\x{80}-\x{F7} a-z0-9@_.\'-]/i', $name)
+    if (preg_match('/[^\x{80}-\x{F7} a-z0-9@+_.\'-]/i', $name)
       || preg_match(
         '/[\x{80}-\x{A0}' .       // Non-printable ISO-8859-1 + NBSP
         '\x{AD}' .                // Soft-hyphen
diff --git a/core/modules/user/src/Tests/UserValidationTest.php b/core/modules/user/src/Tests/UserValidationTest.php
index 81a9e48..9d68578 100644
--- a/core/modules/user/src/Tests/UserValidationTest.php
+++ b/core/modules/user/src/Tests/UserValidationTest.php
@@ -53,6 +53,7 @@ function testUsernames() {
       'foo@example.com'        => array('Valid username', 'assertNull'),
       'foo@-example.com'       => array('Valid username', 'assertNull'), // invalid domains are allowed in usernames
       'þòøÇߪř€'               => array('Valid username', 'assertNull'),
+      'foo+bar'                => array('Valid username', 'assertNull'), // '+' symbol is allowed
       'ᚠᛇᚻ᛫ᛒᛦᚦ'                => array('Valid UTF8 username', 'assertNull'), // runes
       ' foo'                   => array('Invalid username that starts with a space', 'assertNotNull'),
       'foo '                   => array('Invalid username that ends with a space', 'assertNotNull'),