sed convert 4 spaces to 2

Question

How does one convert 4 spaces into 2 spaces with sed? Is it possible?

I found this but it converts tab to spaces:

sed -r ':f; s|^(\t*)\s{4}|\1\t|g; t f' file

Answer 1

The script you posted converts 4*n spaces to n tabs, only if those spaces are preceded only by tabs.

If you want to replace 4 spaces by 2 spaces, but only in indentation, while it's possible to do it with sed, I recommend Perl instead.

perl -ne 's{^( {4}*)}{"  " x (2*len($1)/4)}e' file

In sed:

sed -e 's/^/~/' -e ': r' -e 's/^\( *\)~    /\1  ~/' -e 't r' -e 's/~//' file

You may want to use indent instead.

Answer 2

Doesn't the straight forward way work:

sed -r 's/ {4}/  /g'

If not, post some input where it fails.

Answer 3

If only leading spaces are to be converted:

sed 'h;s/[^ ].*//;s/    /  /g;G;s/\n *//'

Also look at vim's 'ts' setting and :retab command