Can't Get [A-Z] To Work With Regex

I'm trying to use a "Format Value" action to change the filename. %artist% contains the phrase "Artist Name".

If I use $regexp(%artist%,\u,_) I get _rtist _ame.flac as expected.
If I use $regexp(%artist%,[A-Z],_) I get _A_r_t_i_s_t_ _N_a_m_e_.flac.
No idea what I'm doing wrong, can anyone help?

In the end I'd like to to change all characters to underscores except A-Z, a-z, 0-9 and spaces. I'm trying to write this regular expression because if I use \W it doesn't change characters like ö.

EDIT:

Never mind! I was lying awake in bed at 5.15 this morning thinking about it and I suddenly realised I need to escape the square brackets '['. Phew! Thanks to anyone who read my post :slight_smile: