PathProcessor.php

<?php

namespace Drupal\url_alter_test;

use Drupal\Core\PathProcessor\InboundPathProcessorInterface;
use Symfony\Component\HttpFoundation\Request;

/**
 * Path processor for url_alter_test.
 */
class PathProcessor implements InboundPathProcessorInterface {
    
    /**
     * {@inheritdoc}
     */
    public function processInbound($path, Request $request) {
        if (preg_match('!^/user/([^/]+)(/.*)?!', $path, $matches)) {
            if ($account = user_load_by_name($matches[1])) {
                $matches += [
                    2 => '',
                ];
                $path = '/user/' . $account->id() . $matches[2];
            }
        }
        // Rewrite community/ to forum/.
        $path = preg_replace('@^/community(.*)@', '/forum$1', $path);
        if ($path == '/url-alter-test/bar') {
            $path = '/url-alter-test/foo';
        }
        return $path;
    }

}