not <char> emits [^.]

[Aloso]

Describe the bug
not <char> produces the incorrect output, since the dot does not match an arbitrary character within character set.

To Reproduce
Open this playground.

Expected behavior
An error message.

[yoav-lavi]

Hey @Aloso, thanks!

Will hopefully be able to look into this soon. I don't imagine this has any actual use (wouldn't "not char" essentially be an empty string?) but it makes sense to return an error.

How's it going with Pomsky?

[Aloso]

wouldn't "not char" essentially be an empty string?

Not quite. A character set always matches exactly one code point (exception: JavaScript regexes can also match a surrogate, which is one half of a UTF-16 code point that uses 4 bytes).

That means that [] does not match any code point, but it also does not match the empty string, because it expects one code point, but matching the code point never succeeds. So it actually matches nothing.

Other regex engines besides JavaScript do not allow []. In ERE and PCRE, []] matches a ] character, which does not need to be escaped with a backslash. A ] directly following the [ is just treated like \]. So if you want to match nothing in PCRE, you can't use [], you have to use [^\s\S].

The problem with translating not <char> to [] or [^\s\S] is that it ignores line breaks. The dot does not match every character, unless you enable multiline mode. Normally, it matches everything except for line breaks. So normally, not <char> should be compiled to \n, but in multiline mode, it should become [].

But I think it doesn't make sense to support that in Melody (or Pomsky), because nobody needs that. That's why I wrote that I'd expect an error message when I write not <char>.

[Aloso]

How's it going with Pomsky?

Good, thank you.

I wrote you an email about that, which I guess you didn't receive. It's not super important, but I'll open an issue for it 🙂

[yoav-lavi]

Thanks for the detailed explanation!

A lot of useful information there.

To clarify, when referring to an empty string I didn't specifically mean if using a character class:

It's true that Melody uses character classes for some instances of not, but that's more of an implementation detail. For instance not <digit> would be \D - So not <char> wouldn't necessarily have to be a class in this case.

It could theoretically translate to something like (?:(?=)) (excluding handling of multiline expressions)

That being said it doesn't seem like something that'd make sense to allow

[yoav-lavi]

Fixed, will be released soon. Thanks!