```bash
for file in in chapter-*.xhtml
do
sed -ir "12s/\b([A-Z])([A-Z]+)/\1\L\2/g;" "$file"
done
```
This `-ir` tells GNU-sed so alter the file in place (-i) and use regexp-extended (-r).
For line 12 substitute from word boundary an uppercase letter (1) followed by multiple uppercase letters (2) with no1 untouched, but the 2nd pattern replaced by lowercase (\L), and to repeat this procedure globally (/g).
Note that this will turn USB to Usb, USA to Usa, UNO to Uno and so on.
For reaching into subdirectories,
```bash
find -name "chapter-*.xhtml" -exec sed -ir "12s/\b([A-Z])([A-Z]+)/\1\L\2/g;" {} ";"
```
Again, GNU-tools (find, sed) are assumed (as default on most Linux systems).
