var ma1ly = document.getElementsByTagName("span");

var zavinac = "ZAVINAC";
var tecka = "TECKA";
var i;
var index1;
var index2;
var ma1l;

for(i = 0; i < ma1ly.length; i++){
  ma1l = ma1ly[i].innerHTML;
  
  index1 = ma1l.indexOf(zavinac);
  index2 = ma1l.indexOf(tecka);
  if(index1 > 0 && index2 > 0){
    ma1ly[i].innerHTML = ma1l.substring(0,index1) + "&#64;"
            + ma1l.substring(index1 + zavinac.length, index2) + "." 
            +ma1l.substring(index2 + tecka.length);
  }
}