Insert html into textnode

inserthtmltonode.js

      var selection = document.getSelection();

      var cursorPos = selection.anchorOffset;

      var oldContent = selection.anchorNode.nodeValue;

      var toInsert = '_BUTTON_';

      var newContent = oldContent.substring(0, cursorPos) + toInsert + oldContent.substring(cursorPos);

      var replacehtml = '<button class="btn btn-default">A button</button>';

      selection.anchorNode.nodeValue = newContent;

       $(document.getSelection().anchorNode.parentElement).contents().each(function() {
        if(this.nodeType == 3) {
            var u = this.nodeValue;
            var reg = /_BUTTON_/g;
            $(this).replaceWith(u.replace(reg,replacehtml));
        }
      });

Thank you dude, very nice. i have a problem here, if i try to insert the HTML in a row that have no one character will give a problem:
Uncaught TypeError: Cannot read property 'substring' of null at append2

It happen because the 'substring' cannot read a null, have anyway to avoid the null?

Thank you so much