I've only ever done it with stackoverflow because:
1. I think they'll be around for a long time, and
2. they have that policy of explaining answers on the page rather than just referring to external links (yes - irony not lost on me).
Any future dev looking at my 3-line validateEmailAddress() function who wants to learn about it can go to the site and read about it, and maybe find there's a newer status quo way of validating email addresses.
I suppose you could argue that I should copy the explanation of what the regexp does into the comment but really it's just asking for it to become redundant and if someone really wants to get into it they'll probably benefit from reading the entire up-to-date discussion.
Admittedly this is a very specific kind of situation, but this doesn't feel unreasonable or unprofessional to me (bearing in mind that I'm not generally a copy-paster, I prefer to write my own code but in the case of things like email validation I think it's more professional to find an established algorithm than it is to roll your own, unless you have a specific reason for doing so).
If I cut and paste some code, even if not from Stack Overflow I'll document it. Partly because I want to attribute the code correctly, but also so that someone later can go back to the original source. The original source may have additional documentation, explanations, assumptions, etc. that people in the future may want to know. Plus it helps to know that this block of code is an external cut and paste and that modifying it should be done more carefully than other code.
This tends to only be for tricky or time-consuming algorithms. RGB to HSL/HSV, is point in a polygon, hash function, etc. Those point-in-polygon functions in particular can be densely optimized to the point that it is not obvious a) how it works, b) what the original algorithm was.
