edit a text file in reverse order?????

i want to edit a text file in reverse order using vb
how do i do that
what i mean is that i have to read a file in reverse order
and whenever and wherever "urstdm" occurs i've to delete the next 5 line say if the file is like

1333444
ssddd

ddddd
sdds
f kkjkkljkj kjlkjljlj jljljljll
ffggg sddsfdf ffdffsdfdfd klklklkl
urstdm
fgfgfg
fdfggg
ghhjjkhk

i need the output to be

1333444
ssddd


fgfgfg
fdfggg
ghhjjkhk


please help me the keyword urstdm always occurs at the starting of a new line.