diff --git a/lib/adminlib.php b/lib/adminlib.php
index 0f86306e1b4..54ea2fd7103 100644
--- a/lib/adminlib.php
+++ b/lib/adminlib.php
@@ -3645,16 +3645,11 @@ class admin_setting_configmixedhostiplist extends admin_setting_configtextarea {
         $entries = explode("\n", $data);
         foreach ($entries as $key => $entry) {
             $entry = trim($entry);
-            // This regex matches any string which:
-            // a) contains at least one non-ascii unicode character AND
-            // b) starts with a-zA-Z0-9 or any non-ascii unicode character AND
-            // c) ends with a-zA-Z0-9 or any non-ascii unicode character
-            // d) contains a-zA-Z0-9, hyphen, dot or any non-ascii unicode characters in the middle.
-            if (preg_match('/^(?=[^\x00-\x7f])([^\x00-\x7f]|[a-zA-Z0-9])([^\x00-\x7f]|[a-zA-Z0-9-.])*([^\x00-\x7f]|[a-zA-Z0-9])$/',
-                $entry)) {
+            // This regex matches any string that has non-ascii character.
+            if (preg_match('/[^\x00-\x7f]/', $entry)) {
                 // If we can convert the unicode string to an idn, do so.
                 // Otherwise, leave the original unicode string alone and let the validation function handle it (it will fail).
-                $val = idn_to_ascii($entry);
+                $val = idn_to_ascii($entry, IDNA_NONTRANSITIONAL_TO_ASCII, INTL_IDNA_VARIANT_UTS46);
                 $entries[$key] = $val ? $val : $entry;
             }
         }
@@ -3672,7 +3667,7 @@ class admin_setting_configmixedhostiplist extends admin_setting_configtextarea {
         foreach ($entries as $key => $entry) {
             $entry = trim($entry);
             if (strpos($entry, 'xn--') !== false) {
-                $entries[$key] = idn_to_utf8($entry);
+                $entries[$key] = idn_to_utf8($entry, IDNA_NONTRANSITIONAL_TO_ASCII, INTL_IDNA_VARIANT_UTS46);
             }
         }
         return implode("\n", $entries);
diff --git a/lib/tests/admintree_test.php b/lib/tests/admintree_test.php
index f6d581d6afa..dd6b93ef0c6 100644
--- a/lib/tests/admintree_test.php
+++ b/lib/tests/admintree_test.php
@@ -323,4 +323,58 @@ class core_admintree_testcase extends advanced_testcase {
         $setting->write_setting('/mm/nn');
         $this->assertSame('', get_config('abc_cde', 'execpath'));
     }
+
+    /**
+     * Test setting for blocked hosts
+     *
+     * For testing the admin settings element only. Test for blocked hosts functionality can be found
+     * in lib/tests/curl_security_helper_test.php
+     */
+    public function test_mixedhostiplist() {
+        $this->resetAfterTest();
+
+        $adminsetting = new admin_setting_configmixedhostiplist('abc_cde/hostiplist', 'some desc', '', '');
+
+        // Test valid settings.
+        $validsimplesettings = [
+            'localhost',
+            "localhost\n127.0.0.1",
+            '192.168.10.1',
+            '0:0:0:0:0:0:0:1',
+            '::1',
+            'fe80::',
+            '231.54.211.0/20',
+            'fe80::/64',
+            '231.3.56.10-20',
+            'fe80::1111-bbbb',
+            '*.example.com',
+            '*.sub.example.com',
+        ];
+
+        foreach ($validsimplesettings as $setting) {
+            $errormessage = $adminsetting->write_setting($setting);
+            $this->assertEmpty($errormessage, $errormessage);
+            $this->assertSame($setting, get_config('abc_cde', 'hostiplist'));
+            $this->assertSame($setting, $adminsetting->get_setting());
+        }
+
+        // Test valid international site names.
+        $valididnsettings = [
+            'правительство.рф' => 'xn--80aealotwbjpid2k.xn--p1ai',
+            'faß.de' => 'xn--fa-hia.de',
+            'ß.ß' => 'xn--zca.xn--zca',
+            '*.tharkûn.com' => '*.xn--tharkn-0ya.com',
+        ];
+
+        foreach ($valididnsettings as $setting => $encodedsetting) {
+            $errormessage = $adminsetting->write_setting($setting);
+            $this->assertEmpty($errormessage, $errormessage);
+            $this->assertSame($encodedsetting, get_config('abc_cde', 'hostiplist'));
+            $this->assertSame($setting, $adminsetting->get_setting());
+        }
+
+        // Invalid settings.
+        $this->assertEquals('These entries are invalid: nonvalid site name', $adminsetting->write_setting('nonvalid site name'));
+        $this->assertEquals('Empty lines are not valid', $adminsetting->write_setting("localhost\n"));
+    }
 }