Thanks!
My final script ended up like this:
when RULE_INIT {
set ::DEBUG 0
set ::CLASSLIST “msdosdn”
\}
when HTTP\_REQUEST \{
set URI [string tolower [HTTP::uri]]
if \{ [CLASS::match $URI contains $::CLASSLIST value] != ""\} \{
HTTP::respond 404 content “Your request was blocked”
if \{ $::DEBUG == 1 \} \{ log "[HTTP::host] found match in URI: $URI" \}
\}
\}