Response when there is no substring matching the group in StringScanner#captures #70

s = StringScanner.new('foobarbaz') #=> #<StringScanner 0/9 @ "fooba...">
s.scan /(foo)(bar)(BAZ)?/  #=> "foobar"
s[0]           #=> "foobar"
s[1]           #=> "foo"
s[2]           #=> "bar"
s[3]           #=> nil
s.captures #=> ["foo", "bar", ""]
s.captures.compact #=> ["foo", "bar", ""]

/(foo)(bar)(BAZ)?/ =~ "foobarbaz" #=> 0
$~.to_a        #=> ["foobar", "foo", "bar", nil]
$~.captures #=> ["foo", "bar", nil]
$~.captures.compact #=> ["foo", "bar"]