function postcodecheck( form )
{
	var cyf = form.cijfers.value
	var let = form.letters.value
	var Validcyf = "0123456789";
	var Validlet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
	var Char

  	if (cyf.length != 4 ) {
	    alert( "De cijfers van de postcode ontbreken." );
	    form.cijfers.focus();
	    return false ;
	}
	for (i = 0; i < cyf.length ; i++) 
      	{ 
      		Char = cyf.charAt(i); 
		if (Validcyf.indexOf(Char) == -1) 
		{
			alert( "Het cijfer gedeelte van de postcode is onjuist." );
			form.cijfers.focus();
			return false ;
		}
      	}

  	if (let.length !=2 ) {
	    alert( "De letters van de postcode ontbreken." );
	    form.letters.focus();
	    return false ;
	}
	for (i = 0; i < let.length ; i++) 
      	{ 
      		Char = let.charAt(i); 
		if (Validlet.indexOf(Char) == -1) 
		{
			alert( "Het letter gedeelte van de postcode is onjuist." );
			form.letters.focus();
			return false ;
		}
      	}
	return true ;
}
function TabNext(obj,len,next_field) {
	var field_length=0;
	if (obj.value.length != field_length) {
		field_length=obj.value.length;
		if (field_length == len) {
			next_field.focus();
		}
	}
}