using System;
using System.Text.RegularExpressions;
using System.IO;
public class Test
{
	public static void Main()
	{
		var str = @"//Successful
[%
  New Comment
%] other content from input

//Match: [%
New Comment
%]

//Fail
[% New Comment %]

//Match: false

//Successfully match single line with
string commentPatt = @""\[%(.*)%\]"";

//Match: [% New Comment %]";
		var rx = new Regex(@"(?s)\[%(.*?)%]");
        var res = rx.Matches(str);
        foreach (Match m in res)
            Console.WriteLine(m.Groups[1].Value);
	}
}