How can I select text between own prefix/suffix?

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C#

selection

RichTextBox

Hey,

I want to select the text between a prefix and a suffix in a richTextBox to show physical formulas.
How can I do this?

What I have tried:

This is the code I need for text formation:

private void highText(RichTextBox rtb, int Start, string Text, int FontSize)
        {
            rtb.SelectionStart = Start;
            rtb.SelectionLength = Text.Length;
            rtb.SelectionCharOffset = 4;
            rtb.SelectionFont = new Font("Microsoft Sans Serif", FontSize);
        }

        private void lowText(RichTextBox rtb, int Start, string Text, int FontSize)
        {
            rtb.SelectionStart = Start;
            rtb.SelectionLength = Text.Length;
            rtb.SelectionCharOffset = -4;
            rtb.SelectionFont = new Font("Microsoft Sans Serif", FontSize);

        }