Improve S5869 description mentioning special characters escapes

[gabriel-vivas-sonarsource]

Label	Description
Rule	S5869 "Duplicate regex chars"
Subject	Add clarification in the description about special character escapes
Community	https://community.sonarsource.com/t/false-positive-for-typescript-s5869-duplicate-regex-chars-with-special-character-classes/98995

Explanation

A community member opened a thread for rule S5869 (duplicate chars in regex character class) thinking there was a false positive:

const result = /^[\r\n\s\t]+/gm.exec(source);

However, the rule is working as expected, since \s includes \r, \n, \t, among other invisible characters.
So the regex could be written this way:

const result = /^\s+/gm.exec(source);

This means the rule is accurate, however, it was not obvious from the rule description.

Understandably, users would expect to see an explicit duplicate character since this is the first example provided:

/[0-99]/ // Noncompliant, this won't actually match strings with two digits
/[0-9.-_]/ // Noncompliant, .-_ is a range that already contains 0-9 (as well as various other characters such as capital letters)

Suggestion

We could improve the rule description to mention that there are special escape characters that work like character classes:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Character_class_escape

• We should mention: \d, \D, \s, \S, \w, \W, since some developers might not be familiar with them
• We could consider adding a specific example for \s, like in the case reported by the user in the community thread.

I think this will make the rule more useful, and help with the learning aspect.