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

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.