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{ a^n b^n | n >= 0}

...
        (a*)
        (??{sprintf("b{%d}", (length($1)))})
       $/x

{ a^n b^n c^n | n >= 0 }

...
        (??{sprintf("b{%d}", (length($1)))})
        (??{sprintf("c{%d}", (length($1)))})
       $/x