nre returns "" instead of nil for missing matches

[flaviut]

let ex2 = "foo".find(re("(?<foo>foo)(?<bar>bar)?"))
doAssert ex2.captures["bar"] == nil

In this example, captures cannot return nil as nil is used to represent the empty string. The distinction between "no match" and "empty match" is significant and useful when it comes to regular expressions.

[timotheecour]

other std libraries had the same issue after the not-nil string change.
what do you suggest for ease of migration?
#8638 would help assess impact of such change

[flaviut]

A possible option for nre might be to have captures[] throw an exception when attempting to access an undefined capture, and provide some sort of API to detect if a capture exists (e.x. "foo" in captures or 4 in captures).

It'd still be necessary to figure out what the correct return type for captures.toSeq (and captures.toTable) would be, although there are still issues with nim-lang/RFCs#512 there. I'm leaning towards returning seq[Option[string]] there, but there might be some other way to do it.

[Araq]

Go for it.