regex - sed one-liner to convert all uppercase to lowercase?

Question

I have a textfile in which some words are printed in ALL CAPS. I want to be able to just convert everything in the textfile to lowercase, using sed. That means that the first sentence would then read, 'i have a textfile in which some words are printed in all caps.'

Answer 1

With tr:

# Converts upper to lower case 
$ tr '[:upper:]' '[:lower:]' < input.txt > output.txt

# Converts lower to upper case
$ tr '[:lower:]' '[:upper:]' < input.txt > output.txt

Or, sed on GNU (but not BSD or Mac as they don't support L or U):

# Converts upper to lower case
$ sed -e 's/(.*)/L1/' input.txt > output.txt

# Converts lower to upper case
$ sed -e 's/(.*)/U1/' input.txt > output.txt