no-invalid-regexp rule doesn't error with example from its documentation

[FloEdelmann]

Tell us about your environment

• ESLint Version: 5.6.0
• Node Version: 8.12.0
• npm Version: 6.3.0

What parser (default, Babel-ESLint, etc.) are you using? default

Please show your full configuration:

Configuration

{
  "env": {
    "es6": true,
    "node": true
  },
  "parserOptions": {
    "ecmaVersion": 2017
  },
  "rules": {
    "no-invalid-regexp": "error"
  }
}

What did you do? Please include the actual source code causing the issue, as well as the command that you used to run ESLint.

// like in the no-invalid-regexp documentation:
// https://github.com/eslint/eslint/blob/master/docs/rules/no-invalid-regexp.md#rule-details
new RegExp('\\');

eslint file.js

What did you expect to happen?

eslint failing with an error from the no-invalid-regexp rule.

What actually happened? Please include the actual, raw output from ESLint.

eslint passed with no output.

[FloEdelmann]

It was still working with eslint 4.15.0. Could it be a regression introduced in #10062?

[sstern6]

Following up on this, can this be reproduced by an eslint member to be a confirmed bug? If so, Ill work on this.

[not-an-aardvark]

Thanks for the report, I can reproduce this issue.

[sstern6]

Thanks @not-an-aardvark ill work on this. Might have some questions.

[platinumazure]

Is this really a bug in the rule? I'm reading the example as a regex which matches against a backslash anywhere in the string.

My understanding is that escaping the backslash would be required in both the RegExp constructor and in a regexp literal:

new RegExp("\\");

/\\/;

If I'm misunderstanding something, I would be grateful to learn what that is. Otherwise, I think this should be a documentation change (to remove the example), not a bugfix.

[not-an-aardvark]

When I run that code, I get an error:

SyntaxError: Invalid regular expression: /\/: \ at end of pattern

[mysticatea]

This is not a bug.

The // is valid Pattern, so new RegExp('\\'); is valid.

Nevermind, I misread it.

[not-an-aardvark]

[sstern6]

@mysticatea im not understanding, how is it a valid pattern when new RegExp("\\"); returns a syntax error?

[mysticatea]

I'm sorry, I misread it as // 😅

[FloEdelmann]

/\/ is an invalid RegExp, as the backslash is not escaped. /\\/ is fine.
In new RegExp('\\'), the backslash is just escaped in the string (in a string, e.g. \n is a valid character), so to escape it in both the string and the RegExp, one has to use new RegExp('\\\\').

[mysticatea]

So this is a bug in regexpp; it hasn't followed the tc39/ecma262#910 spec update yet because I had overlooked it. I will update it.

[sstern6]

@mysticatea I was going to ask if this was a regexpp issue! Thanks for the PR, so when this gets merged in we will need to update the regexpp package and then I was thinking that we should add a test to the test suite to avoid regressions for this use case?

Is this correct?

[not-an-aardvark]

@sstern6 It looks like the fix has already been published as a new version of regexpp. #10920 has the effect of upgrading regexpp and adding a test, as you suggested.

[FloEdelmann]

Thanks for fixing this!