bash - replace differently for different occurences with sed -

bash - replace differently for different occurences with sed -

probably easy question not able figure out. how can utilize sed replace differently different occurrences?

i give example:

a="2012_01_01_05_05_05"

desired output:

mod_a="2012-01-01 05:05:05"

the thing know how sed is:

mod_a=`echo $a | sed 's/_/ /g'`

i removing "_", transforming array, , create new look based on each element, less elegant.

simple sed solution:

sed 's/_/-/;s/_/-/;s/_/ /;s/_/:/g' <<< "$a"

it replaces first , sec _ -, 3rd _ space , remaining ones :.

bash sed