But you can, sometimes, use it in the replacement part.

Sure, but you're not supposed to: Warning on \1 Instead of $1.

But in /([0-9]+){$1}/, the first capture is completed before the quantifier. So, that's not the reason.

Sorry, I don't understand—not the reason for what?

It's a restriction that should have been removed after the regexp engine was no longer recursive.

Sorry, I don't understand this, either. Do you mean ‘re-entrant’? (UPDATE: Nope, just my internals-ignorance revealed. Thanks, ikegami!)