shell - Replacement of single space with sed without touching multiple spaces -

replacement of single space using sed without touching sequences of several spaces.

i have file looks this:

host/domain summary: message delivery  sent cnt  bytes   defers   avg dly max dly host/domain   -------- -------  -------  ------- ------- -----------     937     3293k       0    18.6 s    6.9 m  theory.tifr.res.in      462     3740k       0     7.3 s    9.5 m  ncra.tifr.res.in      286     1053k       4    20.4 s    1.3 h  gmail.com 

question:

i want replace single space comma or without space without touching other spaces. have tried sed directly takes space , replaces them comma.

example: 18.6 s , 6.9 m having single space can replace value give in sed others cannot.

how can done?

it seems funny thing doing, should want do:

sed 's/\([^ ]\) \([^ ]\)/\1,\2/g' 

replace non-blank followed blank followed non-blank first non-blank, comma, , second non-blank. repeat each single blank in line.