possta.innerHTML = "test";

possta.innerText = "test";

<script text="javascript">
      var adressat = document.getElementById("adresa").innerHTML;
      var poss = document.getElementById("posta1").innerHTML;
      var regex = /[0-9]{5}/;
      var match = adressat.match(regex);
           if (poss == "" && match != null) {
               posta1.innerHTML = '||SUBSTR(ent.adress,1,1)||';
               posta2.innerHTML = '||SUBSTR(ent.adress,2,1)||';
               posta3.innerHTML = '||SUBSTR(ent.adress,3,1)||';
               posta4.innerHTML = '||SUBSTR(ent.adress,4,1)||';
               posta5.innerHTML = '||SUBSTR(ent.adress,5,1)||';
           }
</script>