You have to use the SingleLine option:
VB Code:
  1. Dim strPattern As String = "\[B ](.*)\[/B ]"
  2.         Dim linksExpression As New Regex(strPattern, RegexOptions.IgnoreCase Or RegexOptions.Singleline)
  3.         Dim strTemp As String = "[B ]bold" & vbCrLf & "this[/B ]"
  4.         strTemp = linksExpression.Replace(strTemp, "<b>$1</b>")
  5.         MsgBox(strTemp)