//echo -a $regsubex(é,/./g,é)
returning éé

edit.: maybe can be a bug with utf-8
//echo -a $regsubex(é,/./,é) return é©

mIRC is now an unicode application so the string is send to the regex engine in utf-8, you can force the engine to interpret utf8 before matching : //echo -a $regsubex(é,/(*UTF8)./,à)