SC3026 raised for ^ used in sed regex pattern

[serpent7776]

For bugs

• Rule Id (if any, e.g. SC1000): SC3026
• My shellcheck version (shellcheck --version or 'online'): online
• [x] I tried on shellcheck.net and verified that this is still a problem on the latest commit
• [ ] It's not reproducible on shellcheck.net, but I think that's because it's an OS, configuration or encoding issue
• [x] 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
echo "foo--12" | sed -E s/[^[:alnum:]]/_/g

Here's what shellcheck currently says:

Line 2	SC3026: In POSIX sh, ^ in place of ! in glob bracket expressions is undefined.

Here's what I wanted or expected to see:

No warning. I believe this is a false positive, because the pattern in question is a regex, not shell pattern. Also, tried to execute the problematic code under dash and it's working fine.

[brother]

Seems legit. I would probably encapsulate the pattern in quotation marks and thus not trigger the warning.

[serpent7776]

Hmm, there's no errors raised when using quotes. I'd expect that in this case quotes shouldn't change the behaviour though.

[brother]

I'd expect that in this case quotes shouldn't change the behaviour though.

Indeed. And that's why I think the report is correct and the problem is probably with the shellcheck internals.