Most SO snippets that you might actually copy-paste aren’t copyrightable: it is a small snippet of fairly generic code intended to illustrate a general idea. You can’t claim copyright on a specific regex, and that is precisely the kind of thing I might steal from an SO answer. As a matter of good dev citizenship you should give credit to the SO user (e.g. a link in a comment) but it’s almost never a copyright issue. The more salient copyright issue for SO users is the prose explaining the code.