Removing blank lines or replacing white spaces in a file

in vim:

:g/^$/d


:  put into a state for editing with regex commands

g for every line in the file

/  let's start

^  from the start of the line

$   to the end of the line (^$ together is a special instruction that says blank line)

/ next instruction

d delete it

with vi

 :g/^[ ]*$/d

there's a space in between those square brackets 

vim

:g/^\s$/d

\s means white spaces

remove blank lines from a file with sed:
sed -i '/^$/d' myfile.txt

Replace white spaces with commas:

:%s/\s\+/,/g

replace a word in vi

:g;myword;s;;replaceitwiththis;g

For replacing a word after a word see this post

Here is a useful link for regex expressions:

https://www.w3schools.com/jsref/jsref_obj_regexp.asp

for vim:

http://vimdoc.sourceforge.net/htmldoc/pattern.html