SED replace string by occurrence

Shell Programming and Scripting
1

hi all,
I have a text file with following content

PAGENUMBER
asasasa
asasasa
PAGENUMBER
sasasasasa
PAGENUMBER

using sed i want to replace PAGENUMBER by occurrence count

eg

1
asasasa
asasasa
2
sasasasasa
3

Thanks and Regards,
uttam hoode

2

why not use awk, try this:
cat $filename | awk '/PAGENUMBER/{count++; { sub("PAGENUMBER",count, $0)}; }; {print }'

A part of this command is taken from a similar post by lyoncc, thanks to lyoncc.

3 
awk '/PAGENUMBER/{print ++i;next}1' file
4

how can i update the result in the same file? (inline edit)

5

redirect to another file, rename it back to the original.