int GetWordIndex(string word)
{
char[] splitters = new char[]{' '};
string[] wordsArr = RichTextBox1.Text.Split(splitters, StringSplitOptions.RemoveEmptyEntries);
for(int i=0; i< wordsArr.Length; i++)
if(string.Equals(wordsArr[i], word, StringComparison.InvariantCultureIgnoreCase))
return i;
return -1; //indicates there is no match
}

    private void button1_Click(object sender, EventArgs e)
    {
      string find = "blu";
      if (0 <= richTextBox1.Text.IndexOf(find, StringComparison.InvariantCultureIgnoreCase))
      {
        System.Diagnostics.Debugger.Break();
        //found
      }
      else
      {
        // not found
      }
    }

    private void button2_Click(object sender, EventArgs e)
    {
      string input = richTextBox1.Text;
      string[] words = System.Text.RegularExpressions.Regex.Split(input, @"\W+", System.Text.RegularExpressions.RegexOptions.Multiline);
    }