yay! backtracking in action!

$ ./test-regex 'xa*?y' xaaaay
Concat[
  Literal ('x')  
  Quant[
    Literal ('a') 
  ] 
    Literal ('y')  # yep, that indentation is a Rakudo bug
]
     Literal matching "xaaaa": MATCH
       Quant matching "aaaay": MATCH
     Literal matching "aaaay": FAIL
      Concat matching "aaaay": FAIL
     Literal matching "aaaay": MATCH
       Quant matching "aaay«": MATCH
     Literal matching "aaay«": FAIL
      Concat matching "aaay«": FAIL
     Literal matching "aaay«": MATCH
       Quant matching "aay«E": MATCH
     Literal matching "aay«E": FAIL
      Concat matching "aay«E": FAIL
     Literal matching "aay«E": MATCH
       Quant matching "ay«EN": MATCH
     Literal matching "ay«EN": FAIL
      Concat matching "ay«EN": FAIL
     Literal matching "ay«EN": MATCH
       Quant matching "y«END": MATCH
     Literal matching "y«END": MATCH
      Concat matching "«END»": MATCH
 RxContainer matching "«END»": MATCH
<xaaaay @ 0>