C# RichtextBox 如何获得新加入的文本

当有新内容加入事件发生时，我想获取新加入的内容，请问如何获取？怎么获取？

我用winForm做了个，你看下代码吧：
namespace WindowsFormsApplication2
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
string richString = richTextBox1.Text;

string[] richArrayString = richString.Split(' ');
int count = 1;
foreach (string rstr in richArrayString)
{
if (rstr == "fmov" || rstr == "E:\\yoli" || rstr == "E:\\test")
{
string searchString = String.Format("找到的第{0}个相关字符串为：", count) + rstr;
listBox1.Items.Add(searchString);
count++;
}
}
}
private void button2_Click(object sender, EventArgs e)
{
listBox1.Items.Clear();
}
}
}

代码有点多，分两次发给你，前不久写的带查找关键字高亮，希望对你有所帮助： using system; using system.collections.generic; using system.componentmodel; using system.data; using system.drawing; using system.linq; using system.text; using system.windows.forms; using slsoft; using system.text.regularexpressions; namespace txtedit { public partial class selorrep : form { #region 全局变量 edit edit = new edit(); int index = -1; //当前选中文本索引 bool matched = false; //是否匹配 bool highlight = false; //是否全部高亮 #endregion public selorrep(int index) { initializecomponent(); tabcontrol.selectedtab = tabcontrol.tabpages[index]; if (tabcontrol.selectedindex == 0) { this.txb_sel.focus(); } else { this.txb_seltxt.focus(); } this.topmost = true; edit = (edit)this.owner; } private void btn_exit_click(object sender, eventargs e) { this.close(); } private void selorrep_formclosing(object sender, formclosingeventargs e) { reselect(); edit = (edit)this.owner; if (highlight) { btn_light.text = "全部高亮"; btn_light_click(sender, new eventargs()); } edit.array.remove(this.name); }