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

I know that I can write a Ruby case statement to check a match against a regular expressions. However, I'd like to use the match data in my return statement. Something like this semi-pseudocode:

foo = "10/10/2011"

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

How can I achieve that?

Thanks!

Just a note: I understand that I wouldn't ever use a switch statement for a simple case as above, but that is only one example. In reality, what I am trying to achieve is the matching of many potential regular expressions for a date that can be written in various ways, and then parsing it with Ruby's Date class accordingly.