In POSIX sh, ^ in place of ! in glob bracket expressions is undefined.
Problematic code:
echo foo-[^0]*.jpg
Correct code:
echo foo-[!0]*.jpg
or if the intention is to match ^ literally then either do not put it immediately after [ or quote it by backslash \
echo foo-[0^]*.jpg foo-[\^0]*.jpg
Rationale:
[^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]. POSIX explicitly states that behavior of [^c] is unspecified. Dash used to support [^c] when compiled with fnmatch and glob from glibc, but it was considered as a bug and fixed in version 0.5.12. Busybox has a similar bug 14516.
In zsh interactive sessions echo foo-[!0]*.jpg may cause zsh: event not found: 0 error due to attempt of history expansion. BASH however suppresses history expansion when ! is first character in square brackets.
Discrepancy of negation syntax in regular expressions and shell pattern matching appeared in early UNIX versions. Pipeline syntax in shells had not settled yet, some terminals had no | key, so ^ was used there. That is why in some shells exclamation mark ! was chosen instead of circumflex (caret) ^ symbol for negation in globs. For more details related to shell history see "The ^ = | ?" thread in the Unix Heritage Society mailing list or Sven Mascheck. The Traditional Bourne Shell Family. History and Development page.
Exceptions:
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.
Related resources:
- 2.13.1 Patterns Matching a Single Character in the Shell and Utilities (XCU) volume of POSIX.1-2017.
- 0001558: require
[^...]in addition to[!...]for bracket expression negation POSIX enhancement request was rejected. - Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!