Line replace

[DarkBlue]

Hello

  I need some help

  I have a text file which changes dynamically and has
  200-1800 lines. I need to replace a line , this line
  can be located via a text marker like :

  somelines
  # THIS IS MY MARKER
  This is the line to be replaced
  somemorelines
  
  My question is how to do this in place without
  a temporary file , so that the line after
  the marker is being replaced with mynewlinetext.



Thanks
Nx