function validate_required(field,alerttxt) {
  with (field) {
    if (value==null||value=="") { alert(alerttxt);return false }
    else {return true}
  }
}
function validate_checkbox(field) {
  with (field) {
    if (checked==null||checked=="") { return false }
    else { return true }
  }
}
function validate_select(field,alerttxt) {
  with (field) {
    if (value=="-") { alert(alerttxt);return false }
    else { return true }
  }
}

function validate_email(field,alerttxt) {
  with (field) {
    apos=value.indexOf("@")
    dotpos=value.lastIndexOf(".")
    if (apos<1||dotpos-apos<2) 
      {alert(alerttxt);return false}
    else {return true}
  }
}

function validate_referform(thisform) {
  with (thisform) {
    if (validate_required(yourname,"You haven't entered your name")==false) { return false }
    else if (validate_required(friendsname,"You haven't entered your friend's name")==false) { return false }
    else if (validate_required(friendsemail,"You haven't entered your friend's email")==false) { return false }
    else if (validate_email(friendsemail,"\"Friend's email address\" is invalid")==false) { return false }
    else if (validate_checkbox(moreinfo)==true) {
      if (validate_required(youremail,"Please enter your email")==false) { return false }    
      if (validate_email(youremail,"\"Your email address\" is invalid")==false) { return false }
    }
  }
}