function isMail(mailField){
  strMail = mailField.value;
  var re = new RegExp;
  re = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
  var arr = re.exec(strMail);
  if (arr == null)
    return(false);
  else
    return(true);
}

function minLen(txtField, minVal){
  strExp = txtField.value;
  l = strExp.length;
  if (l < minVal)
    return(true);
  else
    return(false);
}

function maxLen(txtField, maxVal){
  strExp = txtField.value;
  l = strExp.length;
  if (l > maxVal)
    return(true);
  else
    return(false);
}

function isBlank(txtField){
  if (txtField.value)
    return (false);
  else
    return(true);
}

function isSelectedZero(txtField){
  selected = txtField.selectedIndex;
  if (selected == 0)
    return(true);
  else
    return(false);
}

function isNumber(txtField){
  numExp = txtField.value;
  if (isNaN(numExp) || (numExp.length == 0))
    return (false);
  else
    return(true);
}

function isCPF(txtField){ 

  var i; 
  s = txtField.value;  
  var c = s.substr(0,9); 
  var dv = s.substr(9,2); 
  var d1 = 0; 
  
  for (i = 0; i < 9; i++){ 
    d1 += c.charAt(i)*(10-i); 
  } 
  
  if (d1 == 0) return false;   
  
  d1 = 11 - (d1 % 11); 
  
  if (d1 > 9) d1 = 0; 
  
  if (dv.charAt(0) != d1) return false; 
  
  d1 *= 2; 
  
  for (i = 0; i < 9; i++){ 
    d1 += c.charAt(i)*(11-i);   
  } 
  
  d1 = 11 - (d1 % 11); 
  
  if (d1 > 9) d1 = 0; 
  
  if (dv.charAt(1) != d1) return false; 
  
  return true; 
  
}


<!-- Begin telefone
var isNN = (navigator.appName.indexOf("Netscape")!=-1);
function autoTab(input,len, e) {
var keyCode = (isNN) ? e.which : e.keyCode; 
var filter = (isNN) ? [0,8,9] : [0,8,9,16,17,18,37,38,39,40,46];
if(input.value.length >= len && !containsElement(filter,keyCode)) {
input.value = input.value.slice(0, len);
input.form[(getIndex(input)+1) % input.form.length].focus();
}
function containsElement(arr, ele) {
var found = false, index = 0;
while(!found && index < arr.length)
if(arr[index] == ele)
found = true;
else
index++;
return found;
}
function getIndex(input) {
var index = -1, i = 0, found = false;
while (i < input.form.length && index == -1)
if (input.form[i] == input)index = i;
else i++;
return index;
}
return true;
}
//  End -->



			<!-- VALIDACAO

			function valida(theForm){
            if (isBlank(theForm.nome)){
              alert("Faltou seu nome!");
              return false;
            }
			else if (isBlank(theForm.email)){
              alert("Por favor coloque seu email!");
              return false;
            }
            if (!isMail(theForm.email)) {
              alert("Por favor coloque um email valido!");
              return false;
            }
			else if (isBlank(theForm.phone)){
              alert("Falta o seu telefone!");
              return false;
			}
			if (isBlank(theForm.cep)){
              alert("Faltou o CEP!");
              return false;
            }            
			else if (isBlank(theForm.mensagem)){
              alert("Falta uma mensagem!");
              return false;
            }                        
            return true;
            }