diff --git a/config/config.default.php b/config/config.default.php
index d7e31ac6..f29d8044 100644
--- a/config/config.default.php
+++ b/config/config.default.php
@@ -266,6 +266,10 @@ return [
     // Whether the mobile number can be shown to other users
     'enable_mobile_show'      => (bool) env('ENABLE_MOBILE_SHOW', false),
 
+    // Regular expression describing a FALSE username.
+    // Per default usernames must only contain alphanumeric chars, "-", "_" or ".".
+    'username_regex' => (string) env('USERNAME_REGEX', '/([^\p{L}\p{N}\-_.]+)/ui'),
+
     // Enables first name and last name
     'enable_user_name'        => (bool) env('ENABLE_USER_NAME', false),
 
diff --git a/includes/model/User_model.php b/includes/model/User_model.php
index b57059d3..ff1d76ed 100644
--- a/includes/model/User_model.php
+++ b/includes/model/User_model.php
@@ -67,27 +67,6 @@ function Users_by_angeltype_inverted(AngelType $angeltype)
         ->get();
 }
 
-/**
- * Strip unwanted characters from a users nick. Allowed are letters, numbers, connecting punctuation and simple space.
- * Nick is trimmed.
- *
- * @param string $nick
- * @return ValidationResult
- */
-function User_validate_Nick($nick)
-{
-    $nick = trim($nick);
-
-    if (strlen($nick) == 0 || strlen($nick) > 24) {
-        return new ValidationResult(false, $nick);
-    }
-    if (preg_match(config('username_regex', '/([^\p{L}\p{N}\-_. ]+)/ui'), $nick)) {
-        return new ValidationResult(false, $nick);
-    }
-
-    return new ValidationResult(true, $nick);
-}
-
 /**
  * Validate the planned arrival date
  *
diff --git a/includes/pages/admin_user.php b/includes/pages/admin_user.php
index 93ac2f09..3d2a23ae 100644
--- a/includes/pages/admin_user.php
+++ b/includes/pages/admin_user.php
@@ -1,6 +1,7 @@
 <?php
 
 use Engelsystem\Config\GoodieType;
+use Engelsystem\Http\Validation\Rules\Username;
 use Engelsystem\Models\Group;
 use Engelsystem\Models\User\User;
 use Illuminate\Database\Query\JoinClause;
@@ -244,9 +245,12 @@ function admin_user()
                 if ($user_source->settings->email_human) {
                     $user_source->email = $request->postData('eemail');
                 }
-                $nickValidation = User_validate_Nick($request->postData('eNick'));
-                if ($nickValidation->isValid()) {
-                    $user_source->name = $nickValidation->getValue();
+
+                $nick = trim($request->get('eNick'));
+                $nickValid = (new Username())->validate($nick);
+
+                if ($nickValid) {
+                    $user_source->name = $nick;
                 }
                 $user_source->save();
 
diff --git a/includes/pages/guest_login.php b/includes/pages/guest_login.php
index 9353b4c0..fd54977c 100644
--- a/includes/pages/guest_login.php
+++ b/includes/pages/guest_login.php
@@ -4,6 +4,7 @@ use Carbon\Carbon;
 use Engelsystem\Database\Database;
 use Engelsystem\Events\Listener\OAuth2;
 use Engelsystem\Config\GoodieType;
+use Engelsystem\Http\Validation\Rules\Username;
 use Engelsystem\Models\AngelType;
 use Engelsystem\Models\Group;
 use Engelsystem\Models\OAuth;
@@ -114,13 +115,13 @@ function guest_register()
         $valid = true;
 
         if ($request->has('username')) {
-            $nickValidation = User_validate_Nick($request->input('username'));
-            $nick = $nickValidation->getValue();
+            $nick = trim($request->get('username'));
+            $nickValid = (new Username())->validate($nick);
 
-            if (!$nickValidation->isValid()) {
+            if (!$nickValid) {
                 $valid = false;
                 $msg .= error(sprintf(
-                    __('Please enter a valid nick.') . ' ' . __('Use up to 24 letters, numbers, connecting punctuations or spaces for your nickname.'),
+                    __('Please enter a valid nick.') . ' ' . __('Use up to 24 letters, numbers or connecting punctuations for your nickname.'),
                     $nick
                 ), true);
             }
@@ -403,7 +404,7 @@ function guest_register()
                     ),
                     form_info(
                         '',
-                        __('Use up to 24 letters, numbers, connecting punctuations or spaces for your nickname.')
+                        __('Use up to 24 letters, numbers or connecting punctuations for your nickname.')
                     ),
                 ]),
 
diff --git a/resources/lang/de_DE/default.po b/resources/lang/de_DE/default.po
index d5e96a22..e4335fe4 100644
--- a/resources/lang/de_DE/default.po
+++ b/resources/lang/de_DE/default.po
@@ -1533,12 +1533,8 @@ msgstr "Gib bitte einen erlaubten Nick an."
 
 #: includes/pages/guest_login.php:77 includes/pages/guest_login.php:277
 #: includes/view/User_view.php:42
-msgid ""
-"Use up to 24 letters, numbers, connecting punctuations or spaces for your "
-"nickname."
-msgstr ""
-"Verwende bis zu 24 Buchstaben, Zahlen, verbindende Schriftzeichen (.-_) oder "
-"Leerzeichen für deinen Nick."
+msgid "Use up to 24 letters, numbers or connecting punctuations for your nickname."
+msgstr "Verwende bis zu 24 Buchstaben, Zahlen oder verbindende Schriftzeichen (.-_) für deinen Nick."
 
 #: includes/pages/guest_login.php:82
 #, php-format
diff --git a/src/Http/Validation/Rules/Username.php b/src/Http/Validation/Rules/Username.php
new file mode 100644
index 00000000..ff9b7cea
--- /dev/null
+++ b/src/Http/Validation/Rules/Username.php
@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Engelsystem\Http\Validation\Rules;
+
+use Respect\Validation\Rules\AbstractRule;
+use Respect\Validation\Validator;
+use RuntimeException;
+
+/**
+ * Username validation.
+ * Usernames must have 1-24 chars and NOT match the regular expression defined under the config key "username_regex".
+ */
+class Username extends AbstractRule
+{
+    public function validate(mixed $input): bool
+    {
+        $regex = config('username_regex');
+
+        if ($regex === null) {
+            throw new RuntimeException('username_regex not set in config');
+        }
+
+        return Validator::length(1, 24)->validate($input)
+            && Validator::not(Validator::regex($regex))->validate($input);
+    }
+}
diff --git a/tests/Unit/Http/Validation/Rules/UsernameTest.php b/tests/Unit/Http/Validation/Rules/UsernameTest.php
new file mode 100644
index 00000000..47b9f84b
--- /dev/null
+++ b/tests/Unit/Http/Validation/Rules/UsernameTest.php
@@ -0,0 +1,71 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Engelsystem\Test\Unit\Http\Validation\Rules;
+
+use Engelsystem\Application;
+use Engelsystem\Config\Config;
+use Engelsystem\Http\Validation\Rules\Username;
+use Engelsystem\Test\Unit\ServiceProviderTest;
+use RuntimeException;
+
+class UsernameTest extends ServiceProviderTest
+{
+    private Username $subject;
+
+    private Config $config;
+
+    public function setUp(): void
+    {
+        $this->subject = new Username();
+
+        $app = $this->getApp([]);
+        $this->config = new Config([]);
+        $app->instance('config', $this->config);
+        $app->get('config');
+        Application::setInstance($app);
+
+        // load "username_regex" from the default config
+        $defaultConfig = include __DIR__ . '/../../../../../config/config.default.php';
+        $this->config->set('username_regex', $defaultConfig['username_regex']);
+    }
+
+    /**
+     * @return array<string,array{string,bool}>
+     */
+    public function provideValidateWithDefaultConfigTestData(): array
+    {
+        return [
+            'empty string' => ['', false],
+            'max length exceeded' => [str_repeat('1', 25), false],
+            'invalid char !' => ['abc!!!', false],
+            'space' => ['ab c', false],
+            'valid Greek letters' => ['λουκάνικο', true],
+            'min length valid' => ['a', true],
+            'valid with all chars' => ['abc123_-.jkl', true],
+            'valid with accents' => ['café', true],
+            'max length valid' => [str_repeat('a', 24), true],
+        ];
+    }
+
+    /**
+     * @covers \Engelsystem\Http\Validation\Rules\Username::validate
+     * @dataProvider provideValidateWithDefaultConfigTestData
+     */
+    public function testValidateWithDefaultConfig(mixed $value, bool $expectedValid): void
+    {
+        self::assertSame($expectedValid, $this->subject->validate($value));
+    }
+
+    /**
+     * @covers \Engelsystem\Http\Validation\Rules\Username::validate
+     */
+    public function testMissingConfigRaisesException(): void
+    {
+        $this->config->set('username_regex', null);
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('username_regex not set in config');
+        $this->subject->validate('test');
+    }
+}