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(parent: 0c9e5d5) | patch

Fix the general case of quantified regex back-references.

Fix the general case of quantified regex back-references.

Cases where a back-reference is part of a larger subexpression that
is quantified have never worked in Spencer's regex engine, because
he used a compile-time transformation that neglected the need to
check the back-reference match in iterations before the last one.
(That was okay for capturing parens, and we still do it if the
regex has *only* capturing parens ... but it's not okay for backrefs.)

To make this work properly, we have to add an "iteration" node type
to the regex engine's vocabulary of sub-regex nodes. Since this is a
moderately large change with a fair risk of introducing new bugs of its
own, apply to HEAD only, even though it's a fix for a longstanding bug.

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