function check_sf_form()
{
    // Init
    var ok_to_submit=1;

    // Check for blank fields
    var check_fields = new Array('first_name','last_name','company','email','phone');
    for (i=0;i<check_fields.length;i++) {
         eval('var check = document.contact_form.'+check_fields[i]);
         if (check.value == "" || check.value == "undefined") {
             ok_to_submit=0;
         }
    }

    // Check for valid email address
    var email = document.contact_form.email;
    var filter = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
    if (ok_to_submit == 1 && !filter.test(email.value)) {
        alert('Sorry, that email address is invalid.');
        return false;
    }

    if (ok_to_submit == 1) {
        // Ok, submit it.
        return true;
    } else {
        // Nope, errors!
        alert("Sorry, you left out a required field.\nPlease check and try again.");
        return false;
    }
}