Yeah, I had a feeling it might be a PCRE quirk as opposed to a mIRC one, but I was hoping the community here would be able to shed some light on it.

After some extensive testing, I've made another discovery. When \k is used, it works as a sort of retarded lookahead. For example:

Code:
/a(\k)abc/


works EXACTLY the same way as:

Code:
/aU(?=.*abc)/