Find function in rich text box without consider case

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace WindowsFormsApplication1
{
    class Class1
    {
        public static void HighlightText(System.Windows.Forms.RichTextBox myRtb, string word, System.Drawing.Color color)
        {
            int s_start = myRtb.SelectionStart, startIndex = 0, index = 0;

            System.Windows.Forms.RichTextBox myRtb1 = new System.Windows.Forms.RichTextBox();
            myRtb1.Text = myRtb.Text.ToLower();
            word = word.ToLower();

            while ((index = myRtb1.Text.IndexOf(word, startIndex)) != -1)
            {
                myRtb.Select(index, word.Length);
                myRtb.SelectionColor = color;

                startIndex = index + word.Length;
            }

            myRtb.SelectionStart = s_start;
            myRtb.SelectionLength = 0;
            myRtb.SelectionColor = System.Drawing.Color.Black;
        }
    }
}

Class1.HighlightText(richTextBox1, "wills", Color.Aqua);

<pre lang="c#">