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Add FAQ entry on differences between v and u #63

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Add FAQ entry on differences between v and u #63

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Add FAQ entry on differences between `v` and `u`
mathiasbynens May 12, 2022
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Expand Up @@ -194,6 +194,52 @@ In other words, the new flag would indicate several connected changes related to

For more discussion see [issue 2](https://github.com/tc39/proposal-regexp-set-notation/issues/2).

### How is the `v` flag different from the `u` flag?

The answer to this question can be useful when “upgrading” existing `u` RegExps to use `v`. Here’s an overview of the differences:

1. Previously invalid patterns making use of the new syntax (see above) now become valid, e.g.

```
[\p{ASCII_Hex_Digit}--[Ff]]
\p{RGI_Emoji}
[_\q{a|bc|def}]
```

- Some previously valid patterns are now errors, specifically those with a character class including either an unescaped [special character](https://arai-a.github.io/ecma262-compare/snapshot.html?pr=2418#prod-ClassSetSyntaxCharacter) `(` `)` `[` `]` `{` `}` `/` `-` `\` `|` or [a double punctuator](https://arai-a.github.io/ecma262-compare/snapshot.html?pr=2418#prod-ClassSetReservedDoublePunctuator):

```
[(]
[)]
[[]
[{]
[}]
[/]
[-]
[|]
[&&]
[!!]
[##]
[$$]
[%%]
[**]
[++]
[,,]
[..]
[::]
[;;]
[<<]
[==]
[>>]
[??]
[@@]
[^^]
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@mathiasbynens mathiasbynens Feb 2, 2023

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@gibson042 @markusicu @macchiati @pthier Is /[^^]/v actually an error? I noticed the current V8 implementation allows it.

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@pthier pthier Feb 2, 2023

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IMO it isn't, as the first ^ is consumed by the production CharacterClass :: [ ^ ClassContents ], so the remaining ^ is not a reserved double punctuator. [^^^] is an error in V8.
Note that also []] is not a breaking change, as this is an error also with /u.

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@mathiasbynens mathiasbynens Feb 2, 2023

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I guess [_^^] or, indeed, [^^^] is a better example. I’ll change this. Thanks!

[]] is currently not listed as a breaking change, so I think there’s no action to be taken for that one. Let me know if I misunderstood.

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@mathiasbynens mathiasbynens Feb 2, 2023

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PR: #74

[``]
[~~]
```

- The `u` flag suffers from confusing case-insensitive matching behavior. The `v` flag has different, improved semantics. See [issue #30](https://github.com/tc39/proposal-regexp-set-notation/issues/30) for details.

### What’s the precedent in other RegExp flavors?

Several other regex engines support some or all of the proposed extensions in some form:
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