Change the first letter of a specific element

Let’s say you want to make all first letters of element with class big-title into red

JS

window.onload = function(){
   var elements = document.getElementsByClassName(".big-title");
   for (var i=0; i<elements.length; i++){
      elements[i].innerHTML = elements[i].innerHTML.replace(/\\b([a-z])([a-z]+)?\\b/gim, "<span class='first-letter'>$1</span>$2");
   }
}

CSS

<style>
.first-letter {
   color: red;
}
</style>

HTML

<div class="big-title">First letter of every word is now red!</div>