function checkContactForm(form) 
{
    var emailExpr, telephoneNumber;
    
    emailExpr       = /^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/;
    telephoneNumber = /[^0123456789]/;//looks for any characters that are not in the given set.
    
    
    for (i = 0; i < form.elements.length; i++) 
    {
        if (form.elements[i].type == "text") 
        {

            if (form.elements[i].value == null || form.elements[i].value == "") 
            {
                alert("Please insert your " + form.elements[i].id);
                form.elements[i].focus();
                return false;
            }
            else if (form.elements[i].id == "email" && !emailExpr.test(form.elements[i].value)) 
            {
                alert("Invalid "+form.elements[i].id +" address");
                form.elements[i].focus();
                return false;
            }
            else if (form.elements[i].id == "telephone" && telephoneNumber.test(form.elements[i].value)) 
            {
                alert("Invalid "+form.elements[i].id+" number");
                form.elements[i].focus();
                return false;
            }
        }
    }
    return true;

}