[SC3003] $'..' is POSIX.1-2024 compliant

[Neved4]

For bugs

• Rule Id: SC3003:
• My shellcheck version: online
• I tried on https://www.shellcheck.net/ and verified that this is still a problem on the latest commit

For new checks and feature suggestions

• https://www.shellcheck.net/ (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

#!/bin/sh
export var=$'\033[0m'

Here's what shellcheck currently says:

export VAR=$'\e[1;32m'
           ^-- [SC3003](https://www.shellcheck.net/wiki/SC3003) (warning): In POSIX sh, $'..' is undefined.

Here's what I wanted or expected to see:

Believe this could be a false positive. POSIX.1-2024 seems to have added this, under §2.2.4 Dollar-Single-Quotes.

Can be found at Austin Group's Bug Tracker: 0000249: Add standard support for $'...' in shell.

Ideally, the current warning should disappear for POSIX shells. Its functionality could be preserved in an optional flag to work with legacy shells.