From developer.mozilla.org:
Syntax
User-Agent: <product> / <product-version> <comment>
Common format for web browsers:
User-Agent: Mozilla/<version> (<system-information>) <platform> (<platform-details>) <extensions>
Directives
<product>
A product identifier
<product-version>
A version number of the product.
<comment>
Zero or more comments containing sub product information, for example.
Examples
Firefox UA string:
Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:47.0) Gecko/20100101 Firefox/47.0
Mozilla/5.0 (Macintosh; Intel Mac OS X x.y; rv:42.0) Gecko/20100101 Firefox/42.0
Chrome UA string:
The Chrome (or Chromium/blink-based engines) user agent string is
similar to the Firefox format. For compatibility, it adds strings like
"KHTML, like Gecko" and "Safari".
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36
Safari UA string:
Mozilla/5.0 (iPhone; CPU iPhone OS 10_3_1 like Mac OS X) AppleWebKit/603.1.30 (KHTML, like Gecko) Version/10.0 Mobile/14E304 Safari/602.1
Internet Explorer UA string:
Mozilla/5.0 (compatible; MSIE 9.0; Windows Phone OS 7.5; Trident/5.0; IEMobile/9.0)