How to rewrite URLs from UPPERCASE to lowercase in .htaccess - regex

Question

I'm trying to assemble a regular expression to rewrite a URL containing uppercase characters to the same URL but in all lowercase.
Example:
example.com/foO-BAR-bAz rewrite to example.com/foo-bar-baz
example.com/FOO-BAR-BAZ rewrite to example.com/foo-bar-baz
example.com/foo-bar-baz does not match
I tried ^\/(?=.*[A-Z]) to match a string with at least one uppercase character but it doesn't match the full string. I also know that I need to use a "capturing group" but I'm not sure how.
I would be implementing this redirect rule in an .htaccess file of an Apache server

Answer 1

If you are on Apache 2.4 then in .htaccess you can use mod_rewrite with an Apache expression and make use of the tolower() function. (This doesn't require the use of a RewriteMap in the server config, as mentioned in comments.) For example:
RewriteEngine On
RewriteCond expr "tolower(%{REQUEST_URI}) =~ /(.*)/"
RewriteRule [A-Z] %1 [L]
The RewriteRule pattern simply checks there is at least one uppercase letter in the requested URL-path. The RewriteCond directive then calls the tolower() function on the URL-path (REQUEST_URI server variable) which is then effectively captured using the regex. The %1 backreference in the substitution string then holds the result of the tolower() function call, ie. the lowercased URL-path, which is internally rewritten to.
To "correct" the URL and issue an external redirect, then just add the R flag to the RewriteRule directive. For example:
:
RewriteRule [A-Z] %1 [R=301,L]
UPDATE: To eliminate a double redirect when redirecting HTTP to HTTPS (and/or non-www vs www) then include the full canonical URL as part of this rule and implement the canonical (scheme + hostname) redirects second.
For example:
# 1 - Upper to lowercase conversion (and HTTPS and WWW)
RewriteCond expr "tolower(%{REQUEST_URI}) =~ /(.*)/"
RewriteRule [A-Z] https://www.example.com%1 [R=301,L]
# 2 - HTTP to HTTPS
:
Note that the %1 backreference already includes the slash prefix at the start of the URL-path, so this is omitted in the substitution string.
HOWEVER, it is not necessarily incorrect to have a double redirect in this situation. ie. Redirect HTTP to HTTPS (same hostname and URL-path) first then canonicalise other elements of the requested URL (hostname, upper/lowercase URL-path etc.) second. These should be edge cases to begin with, so the real-world impact is minimal.
Note that if you are implementing HSTS then it is a requirement that you first redirect from HTTP to HTTPS on the same hostname, before canonicalising the hostname (ie. www vs non-www). In this case you should use the HTTP_HOST server variable (ie. %{HTTP_HOST}) as the hostname in the above redirect. A double redirect cannot be avoided in this scenario.