To parse a URL `/index.php/comment/100/create`, the first rule will apply, which sets the `controller` parameter to
be `comment` and `action` parameter to be `create`. The route `<controller>/<action>` is thus resolved as `comment/create`.
Similarly, to create a URL for the route `comment/index`, the third rule will apply, which creates a URL `/index.php/comments`.
> Info: By parameterizing routes, it is possible to greatly reduce the number of URL rules, which can significantly
improve the performance of [[yii\web\UrlManager|URL manager]].
By default, all parameters declared in a rule are required. If a requested URL does not contain a particular parameter,
or if a URL is being created without a particular parameter, the rule will not apply. To make some of the parameters
optional, you can configure the [[yii\web\UrlRule::defaults|defaults]] property of a rule. Parameters listed in this
property are optional and will take the specified values when they are not provided.
In the following rule declaration, the `page` and `tag` parameters are both optional and will take the value of 1 and
empty string, respectively, when they are not provided.
```php
[
// ...other rules...
[
'pattern' => 'posts/<page:\d+>/<tag>',
'route' => 'post/index',
'defaults' => ['page' => 1, 'tag' => ''],
],
]
```
In the above example, two named parameters are found in the route part of the rules: `controller` and `action`. The former matches a controller ID that's either "post" or "comment", while the latter matches an action ID that could be "create", "update", or "delete". You may name the parameters differently as long as they do not conflict with any GET parameters that may appear in your URLs.
The above rule can be used to parse or create any of the following URLs:
* `/index.php/posts`: `page` is 1, `tag` is ''.
* `/index.php/posts/2`: `page` is 2, `tag` is ''.
* `/index.php/posts/2/news`: `page` is 2, `tag` is `'news'`.
* `/index.php/posts/news`: `page` is 1, `tag` is `'news'`.
Using the above rules, the URL `/index.php/post/123/create` will be parsed as the route `post/create` with `GET` parameter
`id=123`. Given the route `comment/list` and `GET` parameter `page=2`, Yii can create a URL `/index.php/comments?page=2`.
Without using optional parameters, you would have to create 4 rules to achieve the same result.
### Parameterizing Hostnames
### Rules with Server Names <a name="rules-with-server-names"></a>
It is also possible to include hostnames in the rules for parsing and creating URLs. One may extract part of the hostname
to be a `GET` parameter. This is especially useful for handling subdomains. For example, the URL
`http://admin.example.com/en/profile` may be parsed into GET parameters `user=admin` and `lang=en`. On the other hand,
rules with hostnames may also be used to create URLs with parameterized hostnames.
It is possible to include Web server names in the patterns of URL rules. This is mainly useful when your application
should behave differently for different Web server names. For example, the following rules will parse the URL
`http://admin.example.com/login` into the route `admin/user/login` and `http://www.example.com/login` into `site/login`.
