var elem = new Array("topic", "name", "email");
var label = new Array("Choose your topic, ", "Enter your name, ", "Enter your Email address, ");
function checkInput(form) {
    for(i = 0; i < elem.length; i++) {
        if(form.elements[elem[i]].value == "") {
            alert(label[i] + "PLEASE.");
            return false;
        }
    }
    var re = /\S+@\S+\.\S+/;
    if(!form.elements["email"].value.match(re)) {
        alert("Please check your Email Address.");
        return false;
    }
    return true;
}