function valida_campo (field) {

var valid = "aáäbcdeéëfghiíjklmnñoóöpqrstuúüvwxyzßAÁÄBCDEÉËFGHIÍJKLMNÑOÓÖPQRSTUÚÜVWXYZ0123456789.,;:-_¿?¡!' ";
var ok = "yes";
var temp;

for (var i=0; i<field.value.length; i++) {
	temp = "" + field.value.substring(i, i+1);
	if (valid.indexOf(temp) == "-1") ok = "no";
	}
	
	if (ok == "no") {
		alert("Please, use only these characters: aáäbcdeéëfghiíjklmnñoóöpqrstuúüvwxyzßAÁÄBCDEÉËFGHIÍJKLMNÑOÓÖPQRSTUÚÜVWXYZ0123456789 .,;:-_¿?¡!'");
		field.focus();
		field.select();
		return false;
		} else {
			return true;
		}
	
	}