C#正则表达式如何获取字符串前三位和后三位的组合

如题　正则表达式如何获取字符串:“12345 ABC6789”里（中间有空格）的前三位和后三位
组合成 “123789”

 using System;
using System.Text.RegularExpressions;

public class Test
{
    public static void Main()
    {
        // your code goes here
        string s = "12345 ABC6789";
        s = Regex.Replace(s, @"(?<=.{3}).*(?=.{3})", "");
        Console.WriteLine(s);
    }
}

https://ideone.com/oUO4FK

123789