If there is no substring matching the group (s[3]), the behavior is different.
If there is no substring matching the group, the corresponding element (s[3]) should be nil.
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", ""]
FYI
https://docs.ruby-lang.org/ja/latest/method/MatchData/i/captures.html
/(foo)(bar)(BAZ)?/ =~ "foobarbaz" #=> 0
$~.to_a #=> ["foobar", "foo", "bar", nil]
$~.captures #=> ["foo", "bar", nil]
$~.captures.compact #=> ["foo", "bar"]