How to get the occurrence of the selected word in Javascript

Let's say I have a DIV containing text, like so:

<div>
Hello world

test

Hello world

test 2

Hello world
</div>

My function for getting the word when selecting the text is:

function getWord() {
    var txt = document.getSelection();
    var txtRange = txt.getRangeAt(0);
    return txtRange;
}

Let's say I select the middle "Hello world", which is the 2nd occurrence. How do I get the occurrence, which in this case would be 2?

I have the solution:

function getWord() {
    var txt = document.getSelection();
    var txtRange = txt.getRangeAt(0);
    return txtRange;
}

var t = getWord();
var text = document.getElementsByClassName("content")[0];
var precedingRange = document.createRange();
    precedingRange.setStartBefore(text.firstChild);
    precedingRange.setEnd(t.startContainer, t.startOffset);

var textPrecedingSelection = precedingRange.toString();
var count = (textPrecedingSelection.match(new RegExp(t.toString().trim(), 'gi')) || []).length + 1;

alert("Word occurrence: " + count);