Add Username Value Object (#6)

Add Username Value Object

src/Username.php

@@ -0,0 +1,44 @@
+<?php
+
+/*
+ * This file is part of PHP GitHub Profile.
+ *
+ * (c) Anton Komarev <anton@komarev.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace Komarev\GitHubProfileViewsCounter;
+
+use Webmozart\Assert\Assert;
+
+final class Username
+{
+    private const MIN_LENGTH = 1;
+
+    private const MAX_LENGTH = 39;
+
+    /**
+     * RegEx adapted from https://github.com/shinnn/github-username-regex/blob/master/index.js
+     */
+    private const REGEX = '#^[a-z\d](?:[a-z\d]|-(?=[a-z\d])){0,38}$#i';
+
+    private string $username;
+
+    public function __construct(string $username)
+    {
+        Assert::minLength($username, self::MIN_LENGTH);
+        Assert::maxLength($username, self::MAX_LENGTH);
+        Assert::regex($username, self::REGEX, 'Username may only contain alphanumeric characters or single hyphens, and cannot begin or end with a hyphen.');
+
+        $this->username = strtolower($username);
+    }
+
+    public function __toString(): string
+    {
+        return $this->username;
+    }
+}