sed Substitutions

Substitute (find and replace) "good" with "bad" on each line
 sed 's/good/bad/'            + replaces only 1st instance in a line
 sed 's/good/bad/5'           + replaces only 5th instance in a line
 sed 's/good/bad/g'           + replaces all instances in a line
 sed 's/\(.*\)good\(.*good\)/\1bad\2/' + replace the next-to-last case
 sed 's/\(.*\)good/\1bad/'            + replace only the last case

Substitute "good" with "bad" only for lines which contain "bla"
 sed '/bla/s/good/bad/g'
Substitute "good" with "bad" except for lines which contain "bla"
 sed '/bla/!s/good/bad/g'