silly regexp-replace question: how to delete empty lines

[leo]

hi there

in a buffer like

  text is here
  and here and
  here

  further text is here

  an the endtext

i want to delete the two empty lines by an regexp-replace.

so i started with _finding_ the lines. that's easy: "^$" is the regexp. but
obviouly (replace-regexp "^$" "ghdfg" nil nil nil) does nothing.;-)

SO in order to delete empty lines (i.e. to replace it with "") i have to
find the empty line _plus_ the linefeed.

so i tried (replace-regexp "^$
" "ghdfg" nil nil nil) where the linefeed was type as C-q C-j. but it
doesn't find anything.

any idea?

thanks, leo