Simplify nested characters classes and expression character classes

[RunDevelopment]

Motivation
The v flag added set operation to character classes. These set operations can be simplified in some cases.

Description
The rule should apply the following simplifications:

1. [[abc]] -> [abc] and [^[abc]] -> [^abc] and [[^\w&&\d]] -> [^\w&&\d]: remove trivially nested (expression) character classes.
2. [[abc][def]ghi] -> [abcdefghi]: always inline non-negated character classes.
3. [a&&[^b]] -> [a--b] and [[^b]&&a] -> [a--b]: use subtraction
4. [a--[^b]] -> [a&&b]: use intersection
5. [[^a]&&[^b]] -> [^ab]: De Morgan

[ota-meshi]

Thank you for the rule suggestions! I haven't really used nested character classes yet, but I think the rule is very useful!

[RunDevelopment]

I haven't really used nested character classes yet

Same. I only used them when writing tests for v flag regexes thus far. These simplifications are just a suggestion.

I'm looking forward to using the v flag in practice though. I had to write quite a few regexes like /(?!\s)[\w\x80-\uFFFF]/ in the past, and I can now rewrite them, e.g. as /[[\w\P{ASCII}]--\s]/v.

We might even want to make a rule for that. If a regex has the v flag, then transform (?=[x])[y] => [y&&x] and (?![x])[y] => [y--x]. Let me make an issue for that.

[ota-meshi]

I was working on no-useless-character-class rule and noticed that it supports two patterns. #593

• /[[abc]]/v -> /[abc]/v
• /[[^\w&&\d]]/v -> /[^\w&&\d]/v

However, the following patterns are not supported yet. I'm wondering whether we should support them with no-useless-character-class rule or create a separate rule.

• /[^[abc]]/v -> /[^abc]/
• /[[abc][def]ghi]/v -> /[abcdefghi]/v

https://github.com/ota-meshi/eslint-plugin-regexp/pull/593/files#diff-eb1d64f6019f074be64747d51d62981aaddc46eb17bf309a9f11da0789c5fdabR51-R54

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By the way, I think it would be better to use a separate rule to check patterns that rewrite the following expressions. Because I think it's an optimization problem.

• /[a&&[^b]]/v -> /[a--b]/v
• /[a--[^b]]/v -> /[a&&b]/v
• /[[^a]&&[^b]]/v -> /[^ab]/v

[ota-meshi]

However, the following patterns are not supported yet. I'm wondering whether we should support them with no-useless-character-class rule or create a separate rule.

• /[^[abc]]/v -> /[^abc]/
• /[[abc][def]ghi]/v -> /[abcdefghi]/v

I thought for a moment and thought the rule should support them, so I'll change the rule.