find . -name "*.rb" -exec sed 's/oldstring/newstring' {} \;

find . -name "*.yml" | xargs perl -pi~ -e 's/oldstring/newstring/g;'

Essentially they do the same thing, the first uses sed, the second uses Perl. Watch out what you’re doing with these! If you use find . -type f in a directory managed by Subversion, prepare for some headaches.