This might very well be a PCRE bug, this \k behaviour doesnt exist on mIRC 6.3 which doesnt use PCRE 7.7, which the current version uses, but 7.2.
The lookahead behaviour seems abit like \K behaviour
//echo -a $regsubex(hello,/(h\Kello)/,-\1-) $regsubex(hello,/(hello)/,-\1-)
If this isn't a mIRC bug we should probably report it
here as well.