If you also want to remove lines that only contain whitespace, use
resultString = Regex.Replace(subjectString, @"^\s+$[\r\n]*", string.Empty, RegexOptions.Multiline);
^\s+$
will remove everything from the first blank line to the last (in a contiguous block of empty lines), including lines that only contain tabs or spaces.
[\r\n]*
will then remove the last CRLF (or just LF which is important because the .NET regex engine matches the $
between a \r
and a \n
, funnily enough).