using System;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
        string pattern = @"(?s)(?<=^FROM HERE\b.*?)\r?\n\r?\n(?=.*?^TO HERE\b)";
        string substitution = "\n$$$$\n";
        string input = @"3 bla bla! !
 
4 yep yep! ?
 
FROM HERE
 
5 something randdom here!
 
6 perhaps some HTML there
 
TO HERE
 
7 what ever you like over here
 
8 and that's all folks!enter code here";
        RegexOptions options = RegexOptions.Multiline;
 
        Regex regex = new Regex(pattern, options);
        string result = regex.Replace(input, substitution);
        Console.WriteLine(result);
	}
}

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