Wildcards/Globbing: Are character ranges problematic?

Question

In The Linux Command Line William Shotts claims that character ranges can be problematic. See the relevant excerpt below, emphasis is mine.

Character Ranges

If you are coming from another Unix-like environment or have been reading some other books on this subject, you may have encountered the [A-Z] and [a-z] character range notations. These are traditional Unix notations and worked in older versions of Linux as well. They can still work, but you have to be careful with them because they will not produce the expected results unless properly configured. For now, you should avoid using them and use character classes instead.

What is he talking about in the last couple of sentences? What do the POSIX standards say about this?

Answer 1

That most likely refers to locales having uppercase and lowercase characters ordered in alternation, instead of first one, then the other:

$ echo "$LANG"
en_US.UTF-8
$ touch a A z Z
$ ls
A  Z  a  z
$ bash -c 'echo [a-z]'
a A z

However, the appropriate character class works:

$ bash -c 'echo [[:lower:]]'
a z

But might also match more than just a to z:

$ LANG=fi_FI.UTF-8
$ touch ä Ä ö Ö
$ bash -c 'echo [[:lower:]]'
a z ä ö

If you want to avoid that, and only match the English lowercase letters a to z, Bash in particular has an option to interpret the ranges in the ASCII order:

$ bash -c 'shopt -s globasciiranges; echo [a-z]'
a z

And you can always force the default collating order:

$ LC_COLLATE=C bash -c 'echo [a-z]'
a z

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As for what POSIX says, it seems to me that ranges in bracket expressions are left undefined in locales other than the default POSIX one. The pattern matching description refers to the regex description of bracket expressions, which says:

In the POSIX locale, a range expression represents the set of collating elements that fall between two elements in the collation sequence, inclusive. In other locales, a range expression has unspecified behavior: strictly conforming applications shall not rely on whether the range expression is valid, or on the set of collating elements matched.