^in place of
!in glob bracket expressions is undefined.
[^c] is frequently used in most regular expression variants to mean "any character except
c". Ksh and Bash adopted it for globs as well.
However, strictly speaking, the only range complement syntax guaranteed to be supported across shells is
[!c]. Dash only supports it when using
glob from glibc.
If you only intend to target shells that supports this feature, you can change the shebang to a shell that guarantees support, or ignore this warning. Or just rewrite it to be on the technically correct side.
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.