You can't! The reasons are: first, that, case translations for arbitrary length URLs cannot be done via regex patterns and corresponding substitutions. One needs a per-character pattern like the sed/Perl tr|..|..| feature. Second, just making URLs always upper or lower case does not solve the whole problem of case-INSENSITIVE URLs, because URLs actually have to be rewritten to the correct case-variant for the file residing on the filesystem in order to allow Apache to access the file. And the Unix filesystem is always case-SENSITIVE.

But there is a module named mod_speling.c in the Apache distribution. Try this module to help correct people who use mis-cased URLs.