mod_rewrite
documentation at https://httpd.apache.org/docs/2.4/rewrite/intro.html#regex says:
mod_rewrite uses the Perl Compatible Regular Expression vocabulary. In this document, we do not attempt to provide a detailed reference to regular expressions. For that, we recommend the PCRE man pages, the Perl regular expression man page, and Mastering Regular Expressions, by Jeffrey Friedl.
In "PCRE" you have the liberty to match through Unicode properties, see https://perldoc.perl.org/perluniprops#Properties-accessible-through-%5Cp%7B%7D-and-%5CP%7B%7D
Which means, if really properly implemented by mod_rewrite
that \p{Block: Latin_1}
in your expression would match any character in the "Latin 1 Supplement" block, which is the case for characters ÄÖäö
.
There are of course other ways to select, depending on expected input. You have even \p{General_Category: Letter}
for any character that is defined as a "letter" in the Unicode database.
This has been asked several times, but no answer I found works.
When asking, it could help to specify, even quickly, what you tried already that doesn't work. That allows to avoid anyone repeating something you already know not to work. I did not try the above, so maybe it doesn't work (but should based on documentation) or maybe you tried already?