Docs: grep() using RegExp with global flag /g

[juergba]

Description

The global flag /g in a RegExp expression is behaving weirdly with Mocha's --grep option. The selection of tests based on the fullTitle seems wrong as tests get lost, but at the end it's just javascript.

let reIgnore = /a/g;

console.log(reIgnore.test("a")); // true
console.log(reIgnore.test("a")); // false
console.log(reIgnore.test("a")); // true
console.log(reIgnore.test("a")); // false

Description of the Change

Using a flag as /g or /y doesn't make any sense in this context.

• improve Mocha.prototype.grep for better conversion of string into RegExp
• clarify documentation

Alternate Designs

Instead of re-using the same RegExp instance for checking each test, we could construct new instances:

let reIgnore = /a/g;

console.log(new RegExp(reIgnore).test("a")); // true
console.log(new RegExp(reIgnore).test("a")); // true
console.log(new RegExp(reIgnore).test("a")); // true
console.log(new RegExp(reIgnore).test("a")); // true

Mocha's grep selection is running in any case, with the --grep value or with our defaultGrep /.*/. Therefore I decided to not apply any fix for this edge case.

Applicable issues

closes #4704

[outsideris]

<br> is not needed here.

[juergba]

I tried, there is no end-of-line without <br> in our markdown file.