They are both correct.
$replace() performs an in place exchange of characters.
$regsubex() performs a more complex processing of the text that involves conversion back and forth between UTF-8 and Unicode, which involves checks on the validity of the encoding.
A possible solution might be to change mIRC to use Unicode PCRE instead of ANSI PCRE which would mean that UTF-8 conversions would not be needed. However, it is not clear how this would affect existing scripts that pass UTF-8/Unicode in strings to $regsub/$regsubex/etc.
