Windows Forms RichTextBox Change Code Color

RichTextBoxChangeCodeColor.cs

internal static class ProtoChangeColorHandler
{
    private static readonly string[] c_KeyWords =
    {
        "default", "enum", "message", "import", "group", "package",
        "extend", "extensions", "to", "max",
        "service", "rpc", "returns",
        "true", "false",
        "required", "optional", "repeated",
        "double", "float", "int32", "int64", "uint32", "uint64",
        "sint32", "sint64", "fixed32", "fixed64", "sfixed32",
        "sfixed64", "bool", "string", "bytes",
        "option", "packed", "deprecated", "optional"
    };

    internal static void ProtoChangeColor(this RichTextBox richTextBox)
    {
        richTextBox.SelectAll();
        richTextBox.SelectionColor = Color.Black;
        richTextBox.SelectionFont = richTextBox.Font;
        for (int i = 0; i < c_KeyWords.Length; i++)
        {
            ChangeKeyWordsColor(richTextBox, c_KeyWords[i]);
        }
        ChangeAnnotationColor(richTextBox);
        richTextBox.SelectionStart = richTextBox.TextLength;
        richTextBox.Focus();
        richTextBox.SelectionColor = Color.Black;
        richTextBox.SelectionFont = richTextBox.Font;
    }

    private static void ChangeKeyWordsColor(RichTextBox richTextBox, string keyword)
    {
        int searchIndex = 0;
        while (true)
        {
            string text = richTextBox.Text;
            int index = richTextBox.Text.IndexOf(keyword, searchIndex);
            if (index != -1)
            {
                if (index > 0)
                {
                    if (CheckContextLink(text[index - 1]))
                    {
                        searchIndex = index + keyword.Length;
                        continue;
                    }
                }

                if (index + keyword.Length < text.Length)
                {
                    if (CheckContextLink(text[index + keyword.Length]))
                    {
                        searchIndex = index + keyword.Length;
                        continue;
                    }
                }

                richTextBox.Select(index, keyword.Length);
                richTextBox.SelectionColor = Color.Blue;
                searchIndex = index + keyword.Length;
            }
            else
            {
                break;
            }
        }
    }

    private static bool CheckContextLink(char c)
    {
        return c >= 'A' && c <= 'Z' ||
               c >= 'a' && c <= 'z' ||
               c >= '0' && c <= '9' ||
               c == '_';
    }

    private static void ChangeAnnotationColor(RichTextBox richTextBox)
    {
        int searchIndex = 0;
        for (int lineId = 0; lineId < richTextBox.Lines.Length; lineId++)
        {
            int index = richTextBox.Lines[lineId].IndexOf("//", 0);
            if (index != -1)
            {
                richTextBox.Select(searchIndex + index, richTextBox.Lines[lineId].Length - index);
                richTextBox.SelectionColor = Color.Green;
            }
            searchIndex += richTextBox.Lines[lineId].Length + 1;
        }
    }
}