Imports System
Imports System.Text.RegularExpressions
Public Class Program
Public Shared Sub Main()
Dim str As String = "###Word1 Word2" & Environment.NewLine & "Word3" & Environment.NewLine & "Word4 Word5 Word6" & Environment.NewLine & "##Word7" & Environment.NewLine & "#Word8 Word9" & Environment.NewLine
str = Regex.Replace(str, "#+(.*)\n", "_$1_" & Environment.NewLine)
Console.WriteLine(str)
End Sub
End Class
' run:
'
' _Word1 Word2_
' Word3
' Word4 Word5 Word6
' _Word7_
' _Word8 Word9_
'