如何用C#实现richTextBox里面文本的查找功能？

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);
}