function echeck(str) {
  var filter=/^([\w\-\.]+)@((\[([0-9]{1,3}\.){3}[0-9]{1,3}\])|(([\w\-]+\.)+)([a-zA-Z]{2,4}))$/
  return (filter.test(str))
}

function validarContacto (){
  if (document.forms['contacto'].email.value == "") {
    alert ("Debe ingresar su email.");
  } else if (document.forms['contacto'].email.value != "" && echeck(document.forms['contacto'].email.value)==false){
    alert ("El email ingresado no es válido.");
  } else {
    document.forms['contacto'].submit ();
  }
}

function clearText(thefield){
if (thefield.defaultValue==thefield.value)
thefield.value = ""
}