"This one is rather more complex, so let's go through it field
by field, and follow that with some examples. The value of the
'pattern' field is key; it is to this value that aspects of the
client request are compared to see if a particular rule matches or
not. It might contain a string or a regular expression; how it is
interpreted is controlled by the 'mode' field.
"Ordinarily, the value of the user-agent line of the robots.txt
stanza will come directly from the request's 'User-agent' header
field. The 'alias' field in the table provides a means to override
this. For instance, the rule may actually have matched Firefox, but
you can say that it matched Opera.
"The awkwardly-named 'field' field specifies which aspect of the
request is to be matched against the pattern. I have found use only
for the user-agent and the IP address, but there is no reason
others might not be used."