Comment by rntz | Hacker News Reader

Comment by rntz | Hacker News Reader

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You propose:

    (?=((?!exp).)*$)

I don't believe you're right, but can't be sure, because you haven't given a method of translating an arbitrary regular expression containing a negation (?~E) into one without.

For example, how would you translate:

    (?~exp)a.*

which asks for "anything not exp, then an `a`, then anything"?

    (?=((?!exp).)*$).*a.*

is wrong, since it doesn't match "aexp".

    (?=((?!exp).)*$)a.*

is also wrong, for the same reason.

    (?=((?!exp).)*a)a.*

is wrong, because it doesn't match "expba".

EDIT: argh, HN formatting rules about asterisks.

I believe he's saying you'd want:

  (?=((?!exp).)*a.*)

I'm a little dubious that full equivalence is maintained (particularly w.r.t. to e.g. #match), but my brain isn't up to handling convoluted REs this early.

Edit: Another hangup I have is how would you do something like:

  ((?~E1)|E2)E3

Obviously you can distribute by duplicating E3, but then it's pretty clear what the absent operator buys you.

    (?=((?!exp).)*a.*)

is also wrong; it doesn't match "expba", but

    (?~exp)a.*

does.

  /(?=((?!exp).)*a.*)/ =~ "expba"
  # => 1

for me at least (using #match is also truthy, but the matched string is not what I'd guess the absent operator would give).

Huh! Now I am super confused.

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