changing all occurances

how wouldi change all occurances of that to they regardless of the number of times it appears on a line?

Replace First:

# echo "abcabcabcabc" | sed 's/ab//'
cabcabcabc

Replace all on a line:

# echo "abcabcabcabc" | sed 's/ab//g'
cccc

using Perl:

perl -pi -e 's/that/they/g' /foo/bar