Find all files with the extension ".txt" in the current directory and all subdirectories, and for each file, execute a command to replace all uppercase characters with lowercase characters and all lowercase characters with uppercase characters.
find . -name "*.txt" -type f -exec sed -i 's/\([[A-Za-z0-9_.-]*\)/\L\1/g; s/\([[A-Za-z0-9_.-]\)/\U\1/g' {} \;