findText function

findText.js

function findText( rootElement, textToFind ) {
	var walker = document.createTreeWalker( rootElement,
						NodeFilter.SHOW_TEXT,
						null,
						false );

	var texts = [],
		nodes = [];
	walker.firstChild();
	nodes.push( walker.currentNode );
	texts.push( walker.currentNode.textContent );
	while ( walker.nextSibling() ) {
		nodes.push( walker.currentNode );
		texts.push( walker.currentNode.textContent );
	}
	var input = texts.join( '' ),
		index = input.indexOf( textToFind ),
		parts = [];
	if ( index !== -1 ) {
		var left = 0, start, end, startOffset, endOffset, begin,
			textLeft = textToFind;
		for ( var i = 0; i < texts.length; ++i ) {
			left += texts[i].length;
			if ( left > index && start === undefined ) {
				start = i;
				if ( start > 0 ) {
					startOffset = index-texts.slice( 0, start ).join( '' ).length;
				} else {
					startOffset = index;
				}
			}
			if ( left >= index + textToFind.length ) {
				end = i;
				if ( texts[start].length - startOffset >= textToFind.length ) {
					endOffset = startOffset + textToFind.length;
				} else {
					var before = texts.slice( 0, start+1 ).join( '' ).length;
					var reminder = input.substring( before, before+textToFind.length-(texts[start].length-startOffset) );
					if ( end === start + 1 ) {
						endOffset = reminder.length;
					} else {
						var s = 0;
						for ( var j = start+1; j < end; ++j ) {
							s += texts[j].length;
						}
						endOffset = reminder.substring(s).length;
					}
				}
				break;
			}
		}
		var range = document.createRange();
		range.setStart( nodes[start], startOffset );
		range.setEnd( nodes[end], endOffset );
		return range;
	}
}

usage:

var range = findText( document.querySelector( 'body' ), "search string" );
if ( range ) {
    var selection = window.getSelection();
    selection.removeAllRanges();
    selection.addRange( range );
}