# Expr

[`new in v1.8.2`](https://github.com/AlexxIT/go2rtc/releases/tag/v1.8.2)

[Expr](https://github.com/antonmedv/expr) - expression language and expression evaluation for Go.

- [language definition](https://expr.medv.io/docs/Language-Definition) - takes best from JS, Python, Jinja2 syntax
- your expression should return a link of any supported source
- expression supports multiple operation, but:
  - all operations must be separated by a semicolon
  - all operations, except the last one, must declare a new variable (`let s = "abc";`)
  - the last operation should return a string
- go2rtc supports additional functions:
  - `fetch` - JS-like HTTP requests
  - `match` - JS-like RegExp queries

## Fetch examples

Multiple fetch requests are executed within a single session. They share the same cookie.

**HTTP GET**

```js
var r = fetch('https://example.org/products.json');
```

**HTTP POST JSON**

```js
var r = fetch('https://example.org/post', {
    method: 'POST',
    // Content-Type: application/json will be set automatically
    json: {username: 'example'}
});
```

**HTTP POST Form**

```js
var r = fetch('https://example.org/post', {
    method: 'POST',
    // Content-Type: application/x-www-form-urlencoded will be set automatically
    data: {username: 'example', password: 'password'}
});
```

## Script examples

**Two way audio for Dahua VTO**

```yaml
streams:
  dahua_vto: |
    expr:
    let host = 'admin:password@192.168.1.123';

    var r = fetch('http://' + host + '/cgi-bin/configManager.cgi?action=setConfig&Encode[0].MainFormat[0].Audio.Compression=G.711A&Encode[0].MainFormat[0].Audio.Frequency=8000');

    'rtsp://' + host + '/cam/realmonitor?channel=1&subtype=0&unicast=true&proto=Onvif'
```

**dom.ru**

You can get credentials from https://github.com/ad/domru

```yaml
streams:
  dom_ru: |
    expr:
    let camera   = '***';
    let token    = '***';
    let operator = '***';

    fetch('https://myhome.proptech.ru/rest/v1/forpost/cameras/' + camera + '/video', {
      headers: {
        'Authorization': 'Bearer ' + token,
        'User-Agent': 'Google sdkgphone64x8664 | Android 14 | erth | 8.26.0 (82600010) | 0 | 0 | 0',
        'Operator': operator
      }
    }).json().data.URL
```

**dom.ufanet.ru**

```yaml
streams:
  ufanet_ru: |
    expr:
    let username = '***';
    let password = '***';
    let cameraid = '***';

    let r1 = fetch('https://ucams.ufanet.ru/api/internal/login/', {
      method: 'POST',
      data: {username: username, password: password}
    });
    let r2 = fetch('https://ucams.ufanet.ru/api/v0/cameras/this/?lang=ru', {
      method: 'POST',
      json: {'fields': ['token_l', 'server'], 'token_l_ttl': 3600, 'numbers': [cameraid]},
    }).json().results[0];

    'rtsp://' + r2.server.domain + '/' + r2.number + '?token=' + r2.token_l
```

**Parse HLS files from Apple**

Same example in two languages - python and expr.

```yaml
streams:
  example_python: |
    echo:python -c 'from urllib.request import urlopen; import re

    # url1 = "https://devstreaming-cdn.apple.com/videos/streaming/examples/bipbop_16x9/bipbop_16x9_variant.m3u8"
    html1 = urlopen("https://developer.apple.com/streaming/examples/basic-stream-osx-ios5.html").read().decode("utf-8")
    url1 = re.search(r"https.+?m3u8", html1)[0]

    # url2 = "gear1/prog_index.m3u8"
    html2 = urlopen(url1).read().decode("utf-8")
    url2 = re.search(r"^[a-z0-1/_]+\.m3u8$", html2, flags=re.MULTILINE)[0]

    # url3 = "https://devstreaming-cdn.apple.com/videos/streaming/examples/bipbop_16x9/gear1/prog_index.m3u8"
    url3 = url1[:url1.rindex("/")+1] + url2

    print("ffmpeg:" + url3 + "#video=copy")'

  example_expr: |
    expr:

    let html1 = fetch("https://developer.apple.com/streaming/examples/basic-stream-osx-ios5.html").text;
    let url1 = match(html1, "https.+?m3u8")[0];

    let html2 = fetch(url1).text;
    let url2 = match(html2, "^[a-z0-1/_]+\\.m3u8$", "m")[0];

    let url3 = url1[:lastIndexOf(url1, "/")+1] + url2;

    "ffmpeg:" + url3 + "#video=copy"
```

## Comparison

| expr                         | python                     | js                             |
|------------------------------|----------------------------|--------------------------------|
| let x = 1;                   | x = 1                      | let x = 1                      |
| {a: 1, b: 2}                 | {"a": 1, "b": 2}           | {a: 1, b: 2}                   |
| let r = fetch(url, {method}) | r = request(method, url)   | r = await fetch(url, {method}) |
| r.ok                         | r.ok                       | r.ok                           |
| r.status                     | r.status_code              | r.status                       |
| r.text                       | r.text                     | await r.text()                 |
| r.json()                     | r.json()                   | await r.json()                 |
| r.headers                    | r.headers                  | r.headers                      |
| let m = match(text, "abc")   | m = re.search("abc", text) | let m = text.match(/abc/)      |
