No, look-behinds are zero-width so the negative look-behind says "there are not 6 (non-newline) characters in front of the colon".

So we want a colon that has 5 caps in front of it and that doesn't have 6 characters in front of it.

Okay, I side-stepped your question. I don't know why ^ doesn't work in look-behinds. It might be possible to convince someone that this is a bug. But you can't convince me of that because I refuse to patch regex code. (:

        - tye (but my friends call me "Tye")