$regsubex() is far more complex than $replace(). Not only does it depend on PCRE processing, it also involves making multiple conversions back and forth between Unicode and UTF-8, and making internal calls to evaluate the identifier provided.
$regsubex(aa,/(a)/gu,$chr($gettok(55357 56607,\n,32)))
Can you describe what you are expecting to happen in each part of this call?
