[рони]

<script>

var re = /((^\d)?\d{1,3})/g; 
document.write(
    [
        "1234567890123".match(re).join(" "),
        "123456789012345".match(re).join(" "),
        "1234567890123456".match(re).join(" "),
        "123456789012345678".match(re).join(" "),
        "1234567890123456789".match(re).join(" ")
    ].join("<br>")
);

</script>