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
Code:
//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.

$maybe