# Wildcard Subscription
MQTT topic name is a UTF-8 encoded string used for message routing. To provide more flexibility, MQTT supports a hierarchical topic namespace. A topic is typically leveled and separated with a slash `/` between the levels such as `chat/room/1`. A [wildcard subscription](https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Topic_Names_and) is a subscription with a topic filter containing one or more wildcard characters. This allows the subscription to match more than one topic name. This page introduces the usage of two types of wildcards supported in MQTT and how to subscribe to topics containing wildcard characters in EMQX.
::: tip Note
Wildcards can only be used for subscriptions, not for publishing. Additionally, **avoid using wildcard subscriptions on a large number of clients** to prevent potential performance impacts.
:::
## Single-Level Wildcard
`+` (U+002B) is a wildcard character that matches only one topic level. The single-level wildcard can be used at any level in the topic filter, including the first and last levels. Where it is used, it must occupy an entire level of the filter. It can be used at more than one level in the topic filter and can be used in conjunction with the multi-level wildcard. Below are some examples of using the single-level wildcard:
```
"+" is valid
"sensor/+" is valid
"sensor/+/temperature" is valid
"sensor+" is invalid (does not occupy an entire level)
```
If a client subscribes to the topic `sensor/+/temperature`, it will receive messages from the following topics:
```awk
sensor/1/temperature
sensor/2/temperature
...
sensor/n/temperature
```
But it will not match the following topics:
```bash
sensor/temperature
sensor/bedroom/1/temperature
```
## Multi-Level Wildcard
`#` (U+0023) is a wildcard character that matches any number of levels within a topic. When using a multi-level wildcard, it must occupy an entire level and must be the last character of the topic, for example:
```pgsql
"#" is valid, matches all topics
"sensor/#" is valid
"sensor/bedroom#" is invalid (+ or # are only used as a wildcard level)
"sensor/#/temperature" is invalid (# must be the last level)
```
If a client subscribes to the topic `sensor/#`，it will receive messages from the following topics:
```pgsql
sensor
sensor/temperature
sensor/1/temperature
```
## Try Wildcard Subscription with MQTTX Client
This section demonstrates how to use the MQTTX client to create subscriptions to wildcard topics. In this demonstration, you can create one client connection `Demo` as a publisher to publish messages. Then, you can create another client connection as a subscriber. The subscriber subscribes to the following wildcard topics:
- `testtopic/+/temperature`
- `testtopic/#`
:::tip Prerequisites
- Knowledge about MQTT [Wildcards](./mqtt-concepts.md#topic-and-wildcards)
- Basic publishing and subscribing operations using [MQTTX](./publish-and-subscribe.md)
:::
1. Start EMQX and MQTTX Desktop. Click the **New Connection** to create a client connection as a publisher.
- Enter `Demo` in the **Name** field.
- Enter the localhost `127.0.0.1` in **Host** to use as an example in this demonstration.
- Leave other settings as default and click **Connect**.
::: tip
More detailed instructions on creating an MQTT connection are introduced in [MQTTX Desktop](./publish-and-subscribe.md#mqttx-desktop).
:::
2. Click **+** in the **Connections** pane to create another connection as a subscriber. Set **Name** to `Subscriber`.
3. Select `Subscriber` in **Connections** and click **+ New Subscription**. In the pop-up dialogue, input `testtopic/+/temperature` in the **Topic** field. Leave other options as default.
4. Select `Demo` in **Connections**. Type `testtopic/room1/temperature` in the topic field and type the message payload `28 degree` in the message field. Click the send button. Send another message with the same payload to the topic `testtopic/room2/temperature`.
5. Select `Subscriber` in **Connections**. You can see the subscriber receives the two messages with different topics sent by the publisher.
6. Click **+ New Subscription**. In the pop-up dialogue, use the default topic `testtopic/#` in the **Topic** field. Leave other options as default.
7. Select `Demo` in **Connections**. Type `testtopic/bedroom/room1/temperature` in the topic field and type the message payload `28 degree` in the message field. Click the send button.
8. Select `Subscriber` in **Connections**. You can see the message is sent to the subscription `testtopic/#` only.
