如何在string.Format方法中输出大括号（{}）

C#(a.cs)

using System;

public class Test
{
	public void Middle(string start, string end)
	{
		string pat1 = string.Format("(?<={0}).*(?={1})", start, end);
        Console.WriteLine(pat1);

        string pat2 = string.Format("(?<={{0}}).*(?={{1}})", start, end);
        Console.WriteLine(pat2);

        string pat3 = string.Format("(?<={{{0}}}).*(?={{{1}}})", start, end);
        Console.WriteLine(pat3);
	}

	public static void Main(string[] args)
	{
		string start = "12";
		string end = "35";
        Test t = new Test();

		t.Middle(start, end);
	}
}using System;

public class Test