gistfile1.php

<?php

require_once __DIR__.'/autoload.php.dist';

use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
use Symfony\Component\Routing\Matcher\UrlMatcher;
use Symfony\Component\Routing\Matcher\RequestMatcherInterface;
use Symfony\Component\Routing\RequestContext;
use Symfony\Component\Routing\RouteCollection;
use Symfony\Component\Routing\Route;
use Symfony\Component\HttpFoundation\Request;

// This condition should be true for the route to match
$condition = "method in ['GET', 'HEAD'] and request.headers.get('User-Agent') =~ '/firefox/i'";

$routes = new RouteCollection();
// here, I'm adding the condition as a Route option
$routes->add('hello', new Route('/hello/{name}', array(), array(), array('condition' => $condition)));

class RequestUrlMatcher extends UrlMatcher implements RequestMatcherInterface
{
    private $language;

    public function setExpressionLanguage(ExpressionLanguage $language)
    {
        $this->language = $language;
    }

    public function matchRequest(Request $request)
    {
        $this->request = $request;

        return $this->match($request->getPathInfo());
    }

    protected function handleRouteRequirements($pathinfo, $name, Route $route)
    {
        $options = $route->getOptions();

        $status = self::REQUIREMENT_MISMATCH;
        if (!isset($options['condition'])) {
            $status = self::REQUIREMENT_MATCH;
        } else {
            // the variables that can be accessed in the condition expression
            $context = array(
                'method' => $this->context->getMethod(),
                'request' => $this->request,
            );

            // the route matches only if the condition evaluates to true
            if ($this->language->evaluate($options['condition'], $context)) {
                $status = self::REQUIREMENT_MATCH;
            }
        }

        return array($status, null);
    }
}

$context = new RequestContext();
$request = Request::create('/hello/Fabien', 'GET', array(), array(), array(), array('HTTP_USER_AGENT' => 'Firefox/1.0'));
$context->fromRequest($request);
$matcher = new RequestUrlMatcher($routes, $context);
$matcher->setExpressionLanguage(new ExpressionLanguage());
// this is going to match as the method is GET and the browser contains Firefox
var_export($matcher->matchRequest($request));

gistfile2.yml

# corresponding YAML configuration file for the code above
hello:
    path: /hello/{name}
    options:
        condition: "method in ['GET', 'HEAD'] and request.headers.get('User-Agent') =~ '/firefox/i'"

# what we could do if condition would become a first class citizen in Symfony
hello:
    path: /hello/{name}
    condition: "method in ['GET', 'HEAD'] and request.headers.get('User-Agent') =~ '/firefox/i'"