I have changed the regex routines to use Unicode PCRE calls. This will be in the next beta. I implemented this change as a number of #ifdefs to insert the corresponding Unicode calls/variables in various places, bypassing the need for UTF-8 conversions. So it can be reversed easily to the original ANSI PCRE.
This means that the OP's $regsubex() call now works as expected. The change also passes my current 100+ test calls to the regex identifiers, however it will need further testing to ensure backwards compatibility.
That said, this issue will actually be present throughout mIRC because back and forth UTF-8 conversions take place in many routines when switching between Unicode/ANSI. It just happens that it was not necessary with PCRE due to Unicode PCRE calls being available.
