RemoveBBCodeTags.js

function stripBBCode(txt) {
    return txt.replaceAll(/\[((?!\[).)*?\]/g, "");
}
alert(stripBBCode(prompt("What BBCode?")));

/*
    View here {
        https://regex101.com/r/5RtRdv/1
    }
    Context {
        https://scratch.mit.edu/discuss/post/5215574/
    }
    About {
        When viewing an ocular post (https://ocular.jeffalo.net/post/*), BBCode will show up in the page title.
        This regex removes text between two [square brackets] (which is the form of a BBCode tag), but escape a tag if it has another pair of square brackets inside of it.
        (https://en.scratch-wiki.info/wiki/BBCode#Advanced_BBCode)
    }
*/

z.dart

String stripBBCode(String txt) {
    return txt.replaceAll(RegExp(r"\[((?!\[).)*?\]"), "");
}

void main() {
  print(stripBBCode("""[b]Hello world![/b]
[[]This is an escaped tag]"""));
}

https://dartpad.dev/?null_safety=true&id=ff5608beeb8d7af35df3705c45230606