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  1. or download this
    use re 'Debug' => "EXECUTE";

say "xb" =~ /^(?(?!a) (?=bb)).b$/x       ? "yes":"no"; # -> yes (wrong
+)
say "xb" =~ /^(?(?!a) (?{}) (?=bb)).b$/x ? "yes":"no"; # -> no  (corre
+ct)
  2. or download this
    Guessing start of match in sv for REx "^(?(?!a) (?=bb)).b$" against "x
+b"
Guessed: match at offset 0
...
                                  failed...
Match failed
no