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

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:

{
  "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.