## Get into command mode (press Esc key if not already so)
## To replace from 1st line to last line
:1$s/^/replacementstring/
or
:%s/^/replacementstring/
## To replace from current line and next 4 lines (total 5 lines)
:.,+4s/^/replacementstring/
## To replace from current line and previous 4 lines (total 5 lines)
:.,-4s/^/replacementstring/g
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