nested if statements

The "string first" command will return the index of the found string or -1 if it's not found. So valid values or -1 (for not found) and 0 to len-1 if the string is found. If you are converting this to a boolean value for the if condition, then the only thing that would fail would be a value of 0 (or when the string starts with your substring). I don't think this is what you want. I'd specifically check that the return value is != -1.

Also, you are using matches_regex when you don't need to. Since you are just looking for the start of the string, the "starts_with" operator is much more efficient. You should avoid regular expressions unless there is no alternative.

Give this a shot:

when HTTP_REQUEST {
  if { [HTTP::uri] starts_with "/servlet/mike" or
       [HTTP::uri] starts_with "/servlet/chad" } {
    if {-1 != [string first -nocase "deere" [HTTP::host]]} {
      log "deere [HTTP::host]"
      redirect to "http://unavailable.deere.com/index.pl?SiteName=unavailable.deere.com"
    } else {
      log "no deere [HTTP::host]"
      redirect to "http://x.x.x.x/index.pl?SiteName=unavailable.com"
    }
  }
  log "fell thru [HTTP::host]"
}

You also might need a "string tolower" in there for the HTTP::uri check...

-Joe