function highlightDoc() {

  var regex = /\w+ing/;
  var regexStr = "[A-Za-z]+ing";
  var doc = DocumentApp.getActiveDocument();
  var body = doc.getBody();

  // parse for all endings in 'ing'
  var searchResult = body.findText(regexStr);
  while (searchResult !== null) {
    var foundText = searchResult.getElement().asText();
    var start = searchResult.getStartOffset();
    var end = searchResult.getEndOffsetInclusive();

    // Change the background color to yellow
    foundText.setBackgroundColor(start, end, "#ffee00");
    searchResult = body.findText(regexStr, searchResult);
  }
}