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/counts.txt <<'GRP_INPUT_1'
id=7 id=42
none
id=5
GRP_INPUT_1

grep_gnu_binary=$(command -v ggrep || command -v grep)
grep_gnu_version=$("$grep_gnu_binary" --version 2>/dev/null || :)
case "$grep_gnu_version" in
  *"GNU grep"*) ;;
  *) printf 'This example requires GNU grep.\n' >&2; exit 69 ;;
esac
ggrep() { "$grep_gnu_binary" "$@"; }

ggrep -Po '(?<=id=)[0-9]+' data/counts.txt