Is the User-Agent line in robots.txt an exact match or a substring match?

Question

When a crawler reads the User-Agent line of a robots.txt file, does it attempt to match it exactly to its own User-Agent or does it attempt to match it as a substring of its User-Agent?

Everything I have read does not explicitly answer this question. According to another StackOverflow thread it is an exact match.

However, the RFC draft makes me believe that it is a substring match. For example, User-Agent: Google will match "Googlebot" and "Googlebot-News". Here is the relevant quotation from the RFC:

The robot must obey the first record in /robots.txt that contains a User-Agent line whose value contains the name token of the robot as a substring.

Additionally, in the "Order of precedence for user-agents" section of Googlebot's documentation it explains that the user agent for Google Images "Googlebot-Image/1.0" match for User-Agent: googlebot.

I would appreciate any clarity here, and the answer may be more complicated than my question. For example, Eugene Kalinin's robots module for node mentions splitting the User-Agent to get the "name token" on line 29 and matching against that. If this is true, then Googlebot's User-Agent "Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)" will not match User-Agent: Googlebot.

Answer 1

In the original robots.txt specification (from 1994), it says:

User-agent

[…]

The robot should be liberal in interpreting this field. A case insensitive substring match of the name without version information is recommended.

[…]

If and which bots/parsers comply with this is another question and can’t be answered in general.

Answer 2

Every robot does this a little differently. There is really no single reliable way to map the user-agent in robots.txt to the user-agent sent in the request headers. The safest thing to do is to treat them as two separate, arbitrary strings. The only 100% reliable way to find the robots.txt user-agent is to read the official documentation for the given robot.

Edit:

Your best bet is generally to read the official documentation for the given robot, but even this is not 100% accurate. As Michael Marr points out, Google has a robots.txt testing tool that can be used to verify which UA will work with a given robot. This tool reveals that their documentation is inaccurate. Specifically, the page https://developers.google.com/webmasters/control-crawl-index/docs/ claims that their media partner bots respond to the 'Googlebot' UA, but the tool shows that they don't.