Well, this sort of thing can be done with awk script but it's probably going to be a little more maintainable if you stick with sed.

The -E option was really just to allow us to say .{150} without -E we are stuck putting 150 dot characters is it's place.

You could do this quite easily in vi with 150i.<ESC>, but this is pretty bad form and I recon it's worth the overhead to build an environment var with this string of 150 dots in it, let's call it D150:

Code:

D150=$(print "%150s" "" | tr " " ".")
sed 's/^\('$D150'.*\)    \([^ ]\)/\18251\2/' infile

The above script will work in most shells (except probably csh). If your doing a few of these sorts of things in the 1 script you could even write a function to fill n dots:

Code:

function filldot {
    printf "%$1s" "" | tr " " "." 
}
sed 's/^\('$(filldot 150)'.*\)    \([^ ]\)/\18251\2/' infile