var str = "127.0.0.1";
    var matches = str.match(/^(\d+)\.(\d+)\.(\d+)\.(\d+)$/);
    if (matches.length > 0) {
        var a = parseInt(matches[1]);
        var b = parseInt(matches[2]);
        var c = parseInt(matches[3]);
        var d = parseInt(matches[4]);
        if (a >= 1 && a <= 255 &&
            b >= 0 && b <= 255 &&
            c >= 0 && c <= 255 &&
            d >= 0 && d <= 255) {
            alert('IP');
        }
    }