using System;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		var text =
    @"WORD 
    [asdf] ---
    123/\*&
    WORD
    [asdf] ---
    123/\*&
    ENDWORD
    [asdf] ---
    123/\*&";
 
        var pattern = @"[ \t]WORD\b.*(?:\r?\n(?!.*[ \t](?:END)?WORD\b).*)*\r?\n[ \t]+ENDWORD\b";
 
        Regex rgx = new Regex(pattern);
        foreach (Match match in rgx.Matches(text))
        {
            Console.WriteLine(match.Value);
        }
	}
}

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