using System;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		            string text = @"N:
            TEL:+65123345
            END:VCARD
            BEGIN:VENV
            BEGIN:VBODY
            Date:8/11/2013 11:59:00 PM
            thi is a test message
            Hello this is a test message on line 2
            END:VBODY
            END:VENV
            END:VENV
            END:VMSG";
 
            int startindex = text.IndexOf("BEGIN:VBODY") + ("BEGIN:VBODY").Length;//The just start index of Date...
            int length = text.IndexOf("END:VBODY") - startindex;//Length of text till END...
            if (startindex >= 0 && length >= 1)
            {
                string value = text.Substring(startindex, length);//This is the text you need.
                Console.WriteLine(value);
            }
            else
            {
                Console.WriteLine("No match found.");
            }
	}
}

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