How to Change Font Color for definite string on RichTextBox after USING SPLIT

Question

Hello Master / Programmer.
I'm trying to use Split(), after using it, i wanna check if the input on RTB == my point then change font color on RTB. Like this example.

INPUT on RTB : Chelsea is my favorite football club. I like playing football Chelsea is my favorite football club. I like playing football
My point : football.

Then I split the input, then i check the result of split in arr each index.
Finally, found ex : arr[4] and [9] = football

Then, how to change font color on RTB screen like
"Chelsea is my favorite . I like playing club."? 。我喜欢踢俱乐部。”？

This my code example :

 ArrayList arrInput = new ArrayList();
 //if the input Chelsea is my favorite football club. I like playing football
 string allInput = rtbInput.Text; 

 string[] splitString = allInput.Split(new char[] { ' ', '\t', ',', '.'});

 foreach (string s in splitString)
 {
     if (s.Trim() != "")
     {      
          int selectionStart = s.ToLower().IndexOf("football");

          if (selectionStart != -1)
          {
              rtbInput.SelectionStart = selectionStart;
              rtbInput.SelectionLength = ("football").Length;
              rtbInput.SelectionColor = Color.Red;
          }
 }
 //What Next?? Im confused. We know that football on arrInput[4].ToString() and [9]
 //How to change font color on RTB screen when the input == football

Answer 1

You'll need to select football and set the SelectionColor property

foreach (string s in splitString)
{
    string trimmedS = s.Trim();

    if (trimmedS != "")
    {
        int selectionStart = -1;
        if (trimmedS.ToLower == "football") // Find the string you want to color
            selectionStart = allInput.Count;

        allInput.Add(s);

        if (selectionStart != -1)
        {
            rtbInput.SelectionStart = selectionStart; // Select that string on your RTB
            rtbInput.SelectionLength = trimmedS.Length;
            rtbInput.SelectionColor = myCustomColor; // Set your color here
        }
    }
}

Edit:
Alternative

// create your allInput first

int selectionStart = allInput.ToLower().IndexOf("football");
if (selectionStart != -1)
{
    rtbInput.SelectionStart = selectionStart;
    rtbInput.SelectionLength = ("football").Length;
    rtbInput.SelectionColor = myCustomColor;
}

I recommend this second answer, because I'm not sure, whether the color will remain or not, as we continue building the RichTextBox.Text

Edit2:
Yet another alternative

// create your allInput first

Regex regex = new Regex("football", RegexOptions.IgnoreCase); // using System.Text.RegularExpressions;

foreach (Match match in regex.Matches(allInput))
{
    rtbInput.SelectionStart = match.Index;
    rtbInput.SelectionLength = match.Length;
    rtbInput.SelectionColor = myCustomColor;
}