function checkFields(form) {
	if ( isBlank(form.fromName.value) ) {
		window.alert("Por favor, llena tu nombre.");
		form.fromName.focus();
		return false;
	} else if ( !checkEmailAddress(form.fromEmail.value) ) {
      form.fromEmail.value = "";
      window.alert("Por favor, llena tu dirección de correo electrónico.");
      form.fromEmail.focus();
      return false;
	} else if ( isBlank(form.toName.value) ) {
		window.alert("Por favor, llena el nombre de tu amigo/amiga.");
		form.toName.focus();
		return false;
	} else if ( !checkEmailAddress(form.toEmail.value) ) {
      form.toEmail.value = "";
      window.alert("Por favor, llena la dirección de correo electrónica de tu amigo(a).");
      form.toEmail.focus();
      return false;
	}	
	return true;
}

function isBlank(s) {
for( var i=0;i<s.length;i++ ) {
	var c = s.charAt(i);
	if( (c != ' ') && (c != '\n') && (c != '\t') )
		return false;
	}
	return true;
}

function checkEmailAddress( emailStr ) {
	var emailPat=/^(.+)@(.+)$/
	var specialChars="\\(\\)<>@,;:\\\\\\\"\\.\\[\\]"
	var validChars="\[^\\s" + specialChars + "\]"
	var quotedUser="(\"[^\"]*\")"
	var ipDomainPat=/^\[(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})\]$/
	var atom=validChars + '+'
	var word="(" + atom + "|" + quotedUser + ")"
	var userPat=new RegExp("^" + word + "(\\." + word + ")*$")
	var domainPat=new RegExp("^" + atom + "(\\." + atom +")*$")
	var matchArray=emailStr.match(emailPat)
	if (matchArray==null) {
		return false
	}
	var user=matchArray[1]
	var domain=matchArray[2]
	if (user.match(userPat)==null) {
		return false
	}
	var IPArray=domain.match(ipDomainPat)
	if (IPArray!=null) {
		for (var i=1;i<=4;i++) {
			if (IPArray[i]>255) {
				return false
			}
		}
		return true
	}
	var domainArray=domain.match(domainPat)
	if (domainArray==null) {
		return false
	}
	var atomPat=new RegExp(atom,"g")
	var domArr=domain.match(atomPat)
	var len=domArr.length
	if (domArr[domArr.length-1].length<2 || domArr[domArr.length-1].length>3) {
		return false
	}
	// Make sure there's a host name preceding the domain.
	if (len<2) {
		return false
	}
	return true;
}