no-useless-escape false positive on chatacter classes including ]

[YodaDaCoda]

Tell us about your environment

• ESLint Version: v5.16.0
• Node Version: v10.15.2
• npm Version: 5.8.0

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

Please show your full configuration:

Configuration

{
  "extends": [
    "eslint:recommended"
  ],
  "parserOptions": {
    "ecmaVersion": 2018
  },
  "env": {
    "browser": true
  },
  "globals": {
    "jQuery": "readonly"
  },
  "rules": {
    "no-global-assign": "error",
    "indent": ["warn", 2, {"MemberExpression": "off", "CallExpression": {"arguments": "off"}}],
    "linebreak-style": ["error", "unix"],
    "quotes": ["warn", "single"],
    "semi": ["error", "always"]
  }
}

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

var regex = (/[\[]/);

eslint deleteme.js

What did you expect to happen?
The regular expression should be permitted; the closing bracket ] needs to be escaped within a character class, else it signifies the end of the character class.

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

/mnt/data-ext/Work/Clouding Around/deleteme.js
  1:5   error  'regex' is assigned a value but never used  no-unused-vars
  1:16  error  Unnecessary escape character: \[            no-useless-escape

✖ 2 problems (2 errors, 0 warnings)

Are you willing to submit a pull request to fix this bug?
No

[platinumazure]

Hi @YodaDaCoda, thanks for the issue.

According to your example, the opening bracket is escaped, not the closing bracket. The opening bracket would not need to be escaped if it's already in a character class.

[YodaDaCoda]

My apologies, you are correct.

Upon further review, I realised that my IDE was highlighting well beyond the unnecessary escape sequence (because eslint -f json doesn't return an endColumn for the no-unnecessary-escape rule). In my attempt to produce a smaller test case for this report (my regex is ~150 chars), I'd bungled the issue.

Would it be possible to add endColumn to the result from no-unnecessary-escape? If so, should I make a separate issue for that, or change this issue?

Thanks for your assistance, and sorry again for the confusion.

[platinumazure]

@YodaDaCoda No worries. Either way is fine although I think a new issue might be slightly easier for the maintainers if it's not too much trouble. And yes, for something as precisely located as an unnecessary escape, I think we probably could/should include an end location. Thanks for clarifying and for informing us about this UX issue!

[YodaDaCoda]

Created #11629