C#

string wordToSearch = "word";

TextRange tr = new TextRange(this.rtb.Document.ContentStart, 

int startPos = tr.Text.IndexOf(wordToSearch);

this.rtb.Document.ContentEnd);
                        
                       
if (startPos > -1)
{
   int l = this.GetLinesCount(tr.Text, startPosition);
   
  TextPointer startRtb = this.rtb.Document.ContentStart;
  TextPointer start = startRtb.GetPositionAtOffset(startPos + l);
  TextPointer end = start.GetPositionAtOffset(wordToSearch.Length);
  
  this.rtb.Selection.Select(start, end);  
  this.rtb.Selection.ApplyPropertyValue(
                                TextElement.BackgroundProperty, 
                                this.rtb.SelectionBrush);
}

public int GetLinesCount(string str, int startPosition)
{
    int lines = str.Take(startPosition).Count(k => 
                k.Equals((char)13) || k.Equals((char)10));
    return lines + 2;
}