Regulars. Modifier i. Ignore case

Please help with the regular schedule.

Objective: to make sure that the case is not case sensitive.

I heard about the i modifier, but for some reason it does not work. Here are the regulars:

  $regular = '#[регулярка]#iuU'; 

To be honest, about the modifier i and u I just learned from Google. Therefore, if someone explains to me exactly how to use it (I heard about some problems with the encoding), I will be very grateful.

The regular itself works, it is absolutely accurate.

When I try to use iu , nothing happens. Those. Regular work, but the register is still taken into account.