Highlighting keywords in a richtextbox in WPF

I'm making a program which needs to look through a paragraph of text and find how many times a certain keyword/keywords appear. It also has to highlight each of these key words in the text. I have managed to make he interface and it can now track how many times the word appears but I am really stuck for how to highlight where the keywords appear. I will post my code below, any help is greatly appreciated on how to search for and highlight text inside a richtextbox. Since this is in WPF the obvious richtextbox.find() is not avaliable for use.

class TextAnalyser
{
    public int FindNumberOfOccurances(List<string> keywords, string email)
    {
        int occurances = 0;

        foreach (string keyword in keywords)
        {
            occurances += email.ToUpper().Split(new string[] { keyword.ToUpper() }, StringSplitOptions.None).Count() - 1; 
        }


        return occurances;
    }

    public void TurnTextRed(List<string> keywords, string email, RichTextBox TextBox)
    {
        foreach(string keyword in keywords)
        {


        }

    }

    public List<string> ConvertTextToList(string text)
    {
        char[] splitChars = {};

        string[] ArrayText = text.Split( splitChars, StringSplitOptions.RemoveEmptyEntries);

        return ArrayText.ToList<string>();

    }

    public string GetStringFromTextBox(RichTextBox TextBox)
    {


        var textRange = new TextRange(

            TextBox.Document.ContentStart,

            TextBox.Document.ContentEnd
        );


        return textRange.Text;
    }

thats the TextAnalyser class.

MainWindow.cs below:

public partial class MainWindow : Window
{



    public MainWindow()
    {
        InitializeComponent();
    }

    private void AnalyseButton_Click(object sender, RoutedEventArgs e)
    {
        var textTool = new TextAnalyser();


        var keyWords = textTool.ConvertTextToList(textTool.GetStringFromTextBox(WordTextBox).Trim());
        var email = textTool.GetStringFromTextBox(EmailTextBox).Trim();

        int usesOfWord = textTool.FindNumberOfOccurances(keyWords, email);

        Occurances.Text = usesOfWord.ToString();
    }



}