replacing word after multiple spaces

Question

I need to replace a word after multiple spaces :

actually the file contains this string :

local   all             all                                         


        

Answer 1

Try this one:

sed -i.bak -e "s/\(all[ ]\+\)peer/\1md5/g" file

You need to escape ()+ characters. Space could be like: [ ] with \+ you say that it should be more than 0. Replace it with * if in some cases you do not have a space between "all" and "peer"

In the replace part you should use \1 - back-reference to the first found part: all, and only peer will be replaced with md5