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/animals.txt <<'GRP_INPUT_1'
cat
dog
cats
dogs
catfish
GRP_INPUT_1

grep -E '^(cat|dog)$' data/animals.txt