function valid(cadena) 
{
	if(cadena.value.length==0)
		return(false);
	var valid_char = "QWERTYUIOPÑLKJHGFDSAZXCVBNM" + "qwertyuiopñlkjhgfdsazxcvbnm" + "1234567890" + "-_.";
	for (i=0;i<cadena.value.length;i++)
	{

		var check=false;
		for(j=0;j<valid_char.length;j++)
		{
			if(cadena.value.charAt(i)==valid_char.charAt(j))
			{
				check=true;
			}
		}
		if(!check)
		{
			return(false);
		}
	}
	return(true);
}
function valida_login()
{
var user= document.login_form.usuario;
var pass= document.login_form.contrasena;
if(valid(user)==false || valid(pass)==false)
	{
	alert('datos no válidos');
	pass.value='';
	user.focus();
	return(false);
	}
else
	return(true);
}