Update: fix no-invalid-regexp false negatives with no flags specified

[mdjermanovic]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[x] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

Tell us about your environment

• ESLint Version: v7.18.0
• Node Version: v12.18.4
• npm Version: 6.14.6

What parser (default, @babel/eslint-parser, @typescript-eslint/parser, etc.) are you using?

default

Please show your full configuration:

Configuration

module.exports = {
  parserOptions: {
    ecmaVersion: 2015
  }
};

What did you do? Please include the actual source code causing the issue.

Online Demo

/* eslint no-invalid-regexp: error */

new RegExp("\\u{0}*");

new RegExp("\\u{0}*", "");

What did you expect to happen?

2 errors

These regular expressions would be valid with the u flag, but it isn't specified.

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

no errors

What changes did you make? (Give an overview)

Fixed the no-invalid-regexp rule to distinguish cases where the flags are not specified, so it is known that the regex does not have the u flag, from cases where the flags are unknown so the rule reports an error only if the pattern is invalid both with and without the u flag.

Is there anything you'd like reviewers to focus on?

[aladdin-add]

lgtm! 💯