set -euo pipefail
export LC_ALL=C
grep_work=$(mktemp -d "${TMPDIR:-/tmp}/grep-example.XXXXXX")
trap 'rm -rf -- "$grep_work"' EXIT
cd "$grep_work"
mkdir -p data
cat > data/colors.txt <<'GRP_INPUT_1'
blue
Blue
blueberry
sky blue
red
GRP_INPUT_1

grep -E '^[a-z]+$' data/colors.txt