You will need to use . instead of [^n] in your shell regex to match any character.

[^n] is being evaluated as [^n] i.e. any character other than n and your example strings number 2, 3, and 4 have the letter n somewhere after matching :.

This should work for you:

test_subject() {
  local subject="$1"
  local pattern="^(feat|fix|docs|style|refactor|test|chore)([a-zA-Z0-9._-]+): .+$"

  if ! [[ $subject =~ $pattern ]]; then
    echo "Invalid subject: $subject"
  else
    echo "  Valid subject: $subject"
  fi
}

test_subject "chore(gh-actions): add script for commit check"
test_subject "chore(gh-actions): add script for commit checking"
test_subject "feat(ABC-123): add new feature"
test_subject "fix(ABC123): add new feature"
test_subject "fix(ABC123): fix previously added feature"
test_subject "fix(scope): fix bug"

Output:

Valid subject: chore(gh-actions): add script for commit check
Valid subject: chore(gh-actions): add script for commit checking
Valid subject: feat(ABC-123): add new feature
Valid subject: fix(ABC123): add new feature
Valid subject: fix(ABC123): fix previously added feature
Valid subject: fix(scope): fix bug

Bash regexp does not know n to be a new line character. The [^n] is just a [^n] so the lines which your script marks as invalid – have “n” in them (“checking”, “new”).

The new line bash regexp do recognize is actually $ – end of the line.

Another point – your test strings do not have n characters in them, so there is no real need to check for its absence – hence a simple <complex_pattern>: .+ is enough, which would mean that complex pattern ends with colon, space and you have something after that. No real need to end the pattern with $ since it already will go up to the end of a string.

Said that… if your just hit Enter inside the pattern, you will get a string like:

  local pattern="^(feat|fix|docs|style|refactor|test|chore)([a-zA-Z0-9._-]+): [^
]+$"

But bush (or at least some versions of it) will see it as a single string with a n and do a real “no n inside the string”.

It is possible – yes. Is it better than simple .+ – definitely not. But as a funny feature and possible way to confuse people – yes.