anchor to start
try $re
  match '0'
  try $re
    match '0'
    try $re
      fail to find '0'
      group is optional, matched zero times
    $re matched ''
    match '1'
    group is optional, matched one time
  $re matched '01'
  match '1'
  group is optional, matched one time
$re matched '0011'
anchor to end
match complete: matched '0011' from start

##</code><code>##

  $re = qr{0(??{$re})?};

##</code><code>##

  $re = qr{(??{$re})?0};