Split a string of HTML into an array by particular tags

From the comments to the question, this seems to be the task:

I'm taking dynamic markdown that I'm scraping from GitHub. Then I want to render it to HTML, but wrap every title element in a ReactJS <WayPoint> component.

The following is a completely library-agnostic, DOM-API based solution.

function waypointify(html) {
    var div = document.createElement("div"), nodes;

    // parse HTML and convert into an array (instead of NodeList)
    div.innerHTML = html;
    nodes = [].slice.call(div.childNodes);

    // add <waypoint> elements and distribute nodes by headings
    div.innerHTML = "";
    nodes.forEach(function (node) {
        if (!div.lastChild || /^h[1-6]$/i.test(node.nodeName)) {
            div.appendChild( document.createElement("waypoint") );
        }
        div.lastChild.appendChild(node);
    });

    return div.innerHTML;
}

Doing the same in a modern library with less lines of code is absolutely possible, see it as a challenge.

This is what it produces with your sample input:

<waypoint><h1>A</h1></waypoint>
<waypoint><h2>B</h2><p>Foobar</p></waypoint>
<waypoint><h3>C</h3></waypoint>

In your example you can use:

/
  <h   // Match literal <h
  (.)  // Match any character and save in a group
  >    // Match literal <
  .*?  // Match any character zero or more times, non greedy
  <\/h // Match literal </h
  \1   // Match what previous grouped in (.)
  >    // Match literal >
/g

var str = '<h1>A</h1><h2>B</h2><p>Foobar</p><h3>C</h3>'
str.match(/<h(.)>.*?<\/h\1>/g); // ["<h1>A</h1>", "<h2>B</h2>", "<h3>C</h3>"]

But please don't parse HTML with regexp, read RegEx match open tags except XHTML self-contained tags

I'm sure someone could reduce the for loop to put the angle brackets back in but this is how I'd do it.

var html = '<h1>A</h1><h2>B</h2><p>Foobar</p><h3>C</h3>';

//split on ><
var arr = html.split(/></g);

//split removes the >< so we need to determine where to put them back in.
for(var i = 0; i < arr.length; i++){
  if(arr[i].substring(0, 1) != '<'){
    arr[i] = '<' + arr[i];
  }

  if(arr[i].slice(-1) != '>'){
    arr[i] = arr[i] + '>';
  }
}

Additionally, we could actually remove the first and last bracket, do the split and then replace the angle brackets to the whole thing.

var html = '<h1>A</h1><h2>B</h2><p>Foobar</p><h3>C</h3>';

//remove first and last characters
html = html.substring(1, html.length-1);

//do the split on ><
var arr = html.split(/></g);

//add the brackets back in
for(var i = 0; i < arr.length; i++){
    arr[i] = '<' + arr[i] + '>';
}

Oh, of course this will fail with elements that have no content.