Recipe: Replacing strings from a text or file

Replacing pattern or text
=========================

$ sed 's/PATTEN/replace_text/g' filename
or
$ stdin | sed 's/PATTERN/replace_text/g'

$ p=pattern
$ r=replaced
$ echo "line contain pattern" | sed "s/$p/$r/g" 
line contain replaced

Saving replacement in file
==========================

$ sed 's/PATTERN/replacement/' -i filename