You cannot do it the way you try to. And that's because the quantifier is dealt with at "compile-time" (compile-time of the regex). It's the same as that you cannot do: 
    my $op = "+";
    my $result = 3 $op 4;

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If you want to do such a thing, you have to resort to an eval. Luckely, Perl regexes have their own evals, (??{ }) (see earlier in this thread).

In reply to Re: Can I use backreferences as quantifiers in a regex? by JavaFan
in thread Can I use backreferences as quantifiers in a regex? by johnbo

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