Why are capital letters included in a range of lower-case letters in an awk regex?

Question

$ echo ABC | awk '$0 ~ /^[a-b]/'
ABC
$ echo ABC | awk '$0 ~ /^[a-a]/'
$ echo ABC | awk '$0 ~ /^a/'
$ 

You see. /[a-b]/ captures A, but /[a-a]/ or /a/ doesn't. Why?

Answer 1

It is a "locale" problem, I think.

In my locale, it_IT, the following snippet

if [[ a < A ]]; then
  echo "a < A"
elif [[ a > A ]]; then
  echo "a > A"
else
  echo "a = A"
fi

if [[ b < A ]]; then
  echo "b < A"
elif [[ b > A ]]; then
  echo "b > A"
else
  echo "b = A"
fi

shows

a < A
b > A

so that A is (surprisingly) between a and b, so in the range.

Try executing

echo ABC | LC_COLLATE=C awk '$0 ~ /^[a-b]/'

Edit

the following command shows the collating order in your locale:

echo $(LC_COLLATE=C printf '%s\n' {A..z} | sort)

the output on my machine is

` ^ _ [ ] a A b B c C d D e E f F g G h H i I j J k K l L m M n N o O p P q Q r R s S t T u U v V w W x X y Y z Z

(cannot understand from bash's manual page if sequence expressions are expanded in locale collating order or not; it seems not).