User Agents are just "free-text" fields; there is absolutely no standard format for them and any software that makes requests (browsers, bots, scripts, malware etc) is free to send it's user agent string in whatever form it likes.
The WhatIsMyBrowser.com API will give you detailed information about all the ways we know a user agent can be weird, dangerous, spammy or have some kind of a problem.
There are dozens of reasons a user agent can seem "weird" and it's a good indication if the traffic is legitimate or not.
User agents can have malicious fragments that attempt to inject SQL or execute commands.
Some fake user agents have restricted (or naughty!) bits in them. Don't show them to customers or staff.
Even the humble user agent string isn't immune to spammers. Don't show display user agents that have spam fragments in them.
It's likely that most user agent strings you've seen follow a pretty standard format; they open with a Mozilla/5.0 fragment, and then maybe some Windows NT or Android or Macintosh fragments, then a Chrome/, Firefox/ or Safari/ fragment and so on. Or perhaps there's an obvious fragment like Googlebot/2.1.
In other words, while fairly "standard" user agents are still a bit of a mess, they're not that crazy.. right?
You'd be forgiven for thinking that, but actually, User Agent Strings can come in a literally unlimited number of formats, and since they are just strings of text, often generated by malfunctioning or incorrect code (at best) and malicious and dangerous code (at worst), if you are doing anything with user agents, it's important to handle them correctly.
You can also use user agent to glean extra information about who or what is sending the request; this can help you identify traffic which should be blocked or at least investigated further.
All of the paid API plans get full access to these fields to help you identify many, many types of problems with the user agents accessing your systems.
As well as checking them for problems, we also sanitize/normalize/fix user agents too.
Our user agent checks help you and your systems stay safe and free of junk user agents.