Validations for only characters

The below code allows user to enter only the character(lower case or upper case)

function check(df)
{
for (n=0;n<document.form.elements.length-1;n++)
{
if(form.elements[n].name.indexOf("invoiceNo")!=-1) && (form.elements[n].value==""))
{
var flag
var flag="false"
var valid="abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ";
for(i=0;i<form.elements[n].value.length;i++)
{
if(valid.indexOf(form.elements[n].value.charAt(i))==-1)
{
flag="true"
alert("Invalid character in Invoice No.")
form.elements[n].focus();
form.elements[n].select();
return false;
break;
}
}

}
}
}