using System;
using System.Text.RegularExpressions;
class Program
{
static void Main()
{
string str = "###Word1 Word2\nWord3\nWord4 Word5 Word6\n##Word7\n#Word8 Word9\n";
// Replace the matches using Regex.Replace
str = Regex.Replace(str, @"#+(.*)\n", "_$1_\n");
Console.WriteLine(str);
}
}
/*
run:
_Word1 Word2_
Word3
Word4 Word5 Word6
_Word7_
_Word8 Word9_
*/