private void bnt_Search_Click(object sender, RoutedEventArgs e)
{
    TextRange textRange = new TextRange(rich.Document.ContentStart, rich.Document.ContentEnd);

    //clear up highlighted text before starting a new search
    textRange.ClearAllProperties();
    lbl_Status.Content = "";

   //get the richtextbox text
   string textBoxText = textRange.Text;

    //get search text
    string searchText = searchTextBox.Text;

    if (string.IsNullOrWhiteSpace(textBoxText) || string.IsNullOrWhiteSpace(searchText))
    {
        lbl_Status.Content = "Please provide search text or source text to search from";
    }

    else
    {

        //using regex to get the search count
        //this will include search word even it is part of another word
        //say we are searching "hi" in "hi, how are you Mahi?" --> match count will be 2 (hi in 'Mahi' also)

        Regex regex = new Regex(searchText);
        int count_MatchFound = Regex.Matches(textBoxText, regex.ToString()).Count;

        for (TextPointer startPointer = rich.Document.ContentStart;
                    startPointer.CompareTo(rich.Document.ContentEnd) <= 0;
                        startPointer = startPointer.GetNextContextPosition(LogicalDirection.Forward))
        {
            //check if end of text
            if (startPointer.CompareTo(rich.Document.ContentEnd) == 0)
            {
                break;
            }

            //get the adjacent string
            string parsedString = startPointer.GetTextInRun(LogicalDirection.Forward);

            //check if the search string present here
            int indexOfParseString = parsedString.IndexOf(searchText);

            if (indexOfParseString >= 0) //present
            {
                //setting up the pointer here at this matched index
                startPointer = startPointer.GetPositionAtOffset(indexOfParseString);

                if (startPointer != null)
                {
                    //next pointer will be the length of the search string
                    TextPointer nextPointer = startPointer.GetPositionAtOffset(searchText.Length);

                    //create the text range
                    TextRange searchedTextRange = new TextRange(startPointer, nextPointer);

                    //color up 
                    searchedTextRange.ApplyPropertyValue(TextElement.BackgroundProperty, new  SolidColorBrush(Colors.Yellow));

                    //add other setting property

                }
            }
        }

        //update the label text with count
        if (count_MatchFound > 0)
        {
            lbl_Status.Content = "Total Match Found : " + count_MatchFound;
        }
        else
        {
            lbl_Status.Content = "No Match Found !";
        }
    }
}