I think rejecting any $str containing anything which isn't a cipher, comma or dot prior to eval $str should be good enough

Not quite, since it would also accept ",....," and other things like that (one of the things I meant with "other subtle bugs"). Personally I would suggest both a regex like /\A(\d+(?:\.\.\d+)?)(?:,(?1))*\z/ and checking eval for errors. And at that level, the complexity is high enough that something like* tybalt89's solution is probably better.

* Update: By which I mean, something like tybalt89's solution with some added error checking ;-)

In reply to Re^5: loop iterator in a string by haukex
in thread loop iterator in a string by Anonymous Monk

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