s/,.*(cd)?.*,/=$1=/

##</code><code>##

  echo ,abcdefg,abcdefg | perl -pe 's/,.*?(cd)?.*?,/=$1=/'

##</code><code>##

  echo ,abcdefg,abcdefg, | perl -pe 's/,(?:(?!cd).)*(cd)?.*?,/=$1=/'

##</code><code>##

  echo ,abcefg,abcdefg, | perl -pe 's/,(?:(?!cd).)*(cd)?.*?,/=$1=/'