[PavelGor]

$("#login").change(function(){
	login =  $("#login").val();
        var expLogin = /^[a-zA-Z0-9_]+$/g;
	var resLogin =  login.search(expLogin);
	if(resLogin ==  -1){
		$("#login").next().hide().text("Неверный  логин").css("color","red").
fadeIn(400);
		$("#login").removeClass().addClass("inputRed");
		loginStat  = 0;
		buttonOnAndOff();
	}else{
loginStat  = 1;
	buttonOnAndOff();
}

function  buttonOnAndOff(){
	if(loginStat  == 1){
		$("#submit").removeAttr("disabled");
	}else{
		$("#submit").attr("disabled","disabled");
	}
}