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

Here's an alternative approach that gets you the same result but doesn't use a switch. If you put your regular expressions in an array, you could do something like this:

res = [ /pat1/, /pat2/, ... ]
m   = nil
res.find { |re| m = foo.match(re) }
# Do what you will with `m` now.

Declaring m outside the block allows it to still be available after find is done with the block and find will stop as soon as the block returns a true value so you get the same shortcutting behavior that a switch gives you. This gives you the full MatchData if you need it (perhaps you want to use named capture groups in your regexes) and nicely separates your regexes from your search logic (which may or may not yield clearer code), you could even load your regexes from a config file or choose which set of them you wanted at run time.

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