I did some more research.

It looks like US English tends to use `lowercase`, while British English tends to `lower case`, and as an alternative to `lowercase` you can also use `lower-case` when using it as an adjective.

See also https://en.wiktionary.org/wiki/lowercase

So, to wrap up:

- you could use lowercase and uppercase as a noun, as an adjective and as a verb
- you can use lower case and upper case only as a noun
- you can use lower-case and upper-case only as an adjective

If that is true - I am no native English speaker, and Éric does not like to convert them all to single words, it gets a bit tougher.

Some - to me - obvious wrong usages would be:

"All IMAP4rev1 commands are supported by methods of the same name (in lower-case)."
=> in lower case or in lowercase

"All POP3 commands are represented by methods of the same name, in lower-case; most return the response text sent by the server."
=> in lower case or in lowercase

"Wrapper around a file that converts output to upper-case."
=> to upper case or to uppercase

"Return a new UUID, in the format that MSI typically requires (i.e. in curly braces, and with all hexdigits in upper-case)."
=> in upper case or in uppercase

"Hostnames are compared lower case."
=> lower-case or lowercase

Éric, are you ok with my suggested changes or do you want me to close the issue?