window.onload = function()
    {
    document.getElementById("strCNPJ_CPF").onkeypress = function()
    {
    nCaract = document.getElementById("strCNPJ_CPF")

    switch(nCaract.value.length)
    {
    case 3: nCaract.value += ".";
    break;

    case 7: nCaract.value += ".";
    break;

    case 11: nCaract.value += "-";
    break;
    }
    }
    }