How to write a Ruby switch statement (case…when) with regex and backreferences?

Question

The references to the latest regex matching groups are always stored in pseudo variables $1 to $9:

case foo
when /^([0-9][0-9])/
    print "the month is #{$1}"
else
    print "something else"
end

You can also use the $LAST_MATCH_INFO pseudo variable to get at the whole MatchData object. This can be useful when using named captures:

case foo
when /^(?<number>[0-9][0-9])/
    print "the month is #{$LAST_MATCH_INFO['number']}"
else
    print "something else"
end