gistfile1.js

// very basic example and don`t remember api. Sorry

var firstWord = 'hello', lastWord = 'world!';
var firstNode = document.evaluate('//text()[contets(.,"' + firstWord + '")]', root ....).snapshotItem(0);
var lastNode = document.evaluate('//text()[contets(.,"' + lastWord + '")]', root ....).snapshotItem(0);
var range = document.createRange();

range.setStart(firstNode, firstNode.data.indexOf(firstWord));
range.setEnd(lastNode, lastNode.data.lastIndexOf(lastWord) + lastWord.length);