废话不多说，直接上代码

代码做示例使用，具体逻辑自行编写

/// <summary>
/// 正则表达式，截取两个字符串中间的字符串
/// </summary>
/// <param name="source">目标字符串</param>
/// <param name="startStr">起始字符串</param>
/// <param name="endStr">结束字符串</param>
/// <returns>截取的结果</returns>
private string CutOutStr(string source, string startStr="", string endStr="")
{
    
    
    Regex rg;
    if (endStr.Equals(""))  //提取特定字符串开头的内容
    {
    
    
        rg = new Regex("(?<=(" + startStr + "))[.\\s\\S]*");
    }
    else if (startStr.Equals(""))  //提取特定字符串结尾的内容
    {
    
    
        rg = new Regex(".*?(?=(" + endStr + "))");
    }
    else  //提取特定字符串开头与结尾之间的内容
    {
    
    
        rg = new Regex("(?<=(" + startStr + "))[.\\s\\S]*?(?=(" + endStr + "))");
    }
    return rg.Match(source).Value;
}

测试调用

private void button5_Click(object sender, EventArgs e)
{
    
    
    string aaa = "测试一段字符串123456789";
    Console.WriteLine(CutOutStr(aaa, "字", "5"));  //截取"字"与"5"之间的字符串
    Console.WriteLine(CutOutStr(aaa, "字"));    //截取"字"开头的字符串
    Console.WriteLine(CutOutStr(aaa, "", "字"));  截取"字"结尾的字符串
}

输出结果

符串1234
符串123456789
测试一段