perl -pe's/(^|\s)((\d?\d)?(\d)(\d)\d{5}|(\d?\d)?(\d)(\d)\d\d|\d{1,3})(?=\s|$)/$1.($3?"    $3$4G":$4?"  $4.$5G":$6?" $6$7M":$7?" $7.$8M":$2)/ge'

##</code><code>##

/dev/dsk/c0t1d0s1       1.2G    814M    391M    68%    /usr
/dev/dsk/c0t1d0s3       245M     44M    176M    21%    /var
/dev/dsk/c0t1d0s0       424M    187M    194M    49%    /home
##</code><code>##

perl -pe's/(^|\s)((\d*?)\d)(\d)\d\d(\d{3})?(?=\s)/$1.($5&&"   ").($3?"  $2":"$2.$4").($5?"G":"M")/sge'
##</code><code>##

$ echo \
1234 12 12345 123456 1234567 12345678 123456789 | perl -pe'. . .'
1.2M 12   12M   123M    1.2G      12G      123G

##</code><code>##

perl -pe's/(^|\s)((\d*?)\d)(\d)(\d)\d(\d{3})?(?=\s)/$1.($6&&"   ").($3?"  ".($2+($4>4)):$2+($4+($5>4))/10).($6?"G":"M")/ges'



##</code><code>##

perl -pe's/((\d*?)\d)(\d)\d\d(\d{3})? /$1.($2?"  ":".$3").($4?"   G ":"M ")/ge'
##</code><code>##

$ echo '
1234 12 123 12345 123456 7654321 12345678 123456789 ' | perl -pe'... 
1.2M 12 123 12  M 123  M 7.6   G 12     G 123     G 



##</code><code>##

perl -pe's!(^|\s)((\d*?)\d)(\d)\d\d((\d{3})*)(?=\s)!$"x($x=length$5).$1.($3?"  $2":"$2.$4").qw(K M G T)[$x/3]!ges'
##</code><code>##

$ echo \
1234 1234567 123456789012 1234567890123 123456789012345 | perl ...
1.2K    1.2M         123G          1.2T            123T



##</code><code>##

perl -pe's!\d{4,}(?=\s)!sprintf"   "x($x=(-4+length$&)/3).($x*3%3?"  %.0f":"%.1f").qw(M G T)[$x],$&/1024**int$x+1!ge'

##</code><code>##

$ echo '
321 4321 654321 87654321 9876543210 98765432101 987654321012 '|perl...
321 4.2M   639M      84G       9.2T         92T         920T 


##</code><code>##

perl -pe's!(^|\s)(\d{4,})(?=\s)!sprintf"$1%s".(($x=-4+length$2)%3?"  %.0f":"%.1f").(qw(M G T))[$x/=3],"   "x$x,$2/1024**int$x+1!ges'