using System;
using System.Text.RegularExpressions;
 
class Program
{
    public static void Main()
    {
        string text = "<SPAN class=\"m\">&lt;</SPAN><SPAN class=\"t\">text</SPAN><SPAN class=\"m\">&gt;</SPAN><SPAN class=\"tx\">ТЕКСТ КОТОРЫЙ НУЖНО ВЫРЕЗАТЬ</SPAN><SPAN class=\"m\">&lt;/</SPAN><SPAN class=\"t\">text</SPAN><SPAN class=\"m\">&gt;</SPAN>";
        string pattern = "(?<=<SPAN class=\"tx\">).*?(?=</SPAN>)";
        Match match = Regex.Match(text, pattern);
        if (match.Success) Console.WriteLine(match.Value);
    }
}

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