This answer gives the reciprocal of the required output, rather than the required output itself.
In a simpler challenge this might cut out a significant part of the work, but for this particular challenge the bulk of the work is still being done. However, this still gives the answerer an advantage over a competitor using the same language but giving the correct required output.
As pointed out in a since deleted comment, the limitations imposed by pure regex mean that answers generally take unary input and give unary output (where arithmetic is required), which doesn't allow an obvious way of giving a fraction as an output, which the challenge in question requires. I think the compromise found, of using unary to express a reciprocal, is ingenious, but I'd like to see discussion of whether this is valid, so we can have consensus one way or the other.
Is such an answer valid?